There also no exceptions for the bumper to extend inside the frame perimeter, but bumper brackets do that as well. The rules about robust bumper attachments are pretty vague.
There’s no requirement for BUMPERS to be outside the frame perimeter, so I don’t know why there would be exceptions for it.
The only requirement related to that is R410, which is very specific - it doesn’t restrict BUMPERS, but rather the BUMPER wood/backing material - other parts of the bumpers (like mounting brackets) are not restricted by that rule.
You’re right that the rules related to the attachment system are vague, allowing teams to create an attachment system that works with their design. But R408 very clearly includes the “rigid fastening system” as part of the BUMPER itself.
I totally agree with your conclusion @Jon_Stratis , if not exactly the reference to R408:
R408 G ends (emphasis mine):
All removable fasteners (e.g. bolts, locking pins, pip-pins, etc.) will be considered part of the BUMPERS.
As such, this is not applicable to the flanges in question, except through some logical thinking that if the removable parts are part of the BUMPER, the parts which are not [intentionally] removable from the main bumper structure must be part of the BUMPER as well.
More directly applicable to this flange is the blue box for 407:
If a multi-part attachment system is utilized (e.g. interlocking brackets on the ROBOT and the BUMPER), then the elements permanently attached to the ROBOT will be considered part of the ROBOT, and the elements attached to the BUMPERS will be considered part of the BUMPER. Each element must satisfy all applicable rules for the relevant system.
That last sentence even makes it clear that this blue box does not only apply to R407, but the other rules, including R402 (Bumpers must stay low).
And OBTW, I’d consider a flange, a tube, and a bolt holding them together to be “a multi-part attachment system”.
I think you mean R407.
Regardless, R408 states “BUMPERS must be constructed as follows (see Figure 9-7):” - a standard reading of that leads to the understanding that everything that follows is considered part of the bumper. Part G stats:
must attach to the FRAME PERIMETER of the ROBOT with a rigid fastening system to form a tight, robust connection to the main structure/frame (e.g. not attached with hook-andloop tape, tape, or cable ties). The attachment system must be designed to withstand vigorous game play. All removable fasteners (e.g. bolts, locking pins, pip-pins, etc.) will be considered part of the BUMPERS.
This part essentially says the same thing as the blue box for R407 (except for the part about items permanently attached to the robot) - except this is part of the actual bumper construction, not a blue box included to help explain the bumper weight limit. It’s a good thing these two places agree 
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My team is using a very similar system this year and we are very happy. We put the brackets on the ends of bumper sections in 1” bumper gaps, which avoids the height issue. No issues with inspection. We actually use aluminum and 3dprinted carriers to hold regular lock nuts rather than rib nuts with we have had issues replacing in the past.
I’ll grab some pictures tomorrow night but I would be happy to show anyone who stops by the 3357 pit in Houston.
You have a neat design for the connection. I think Jon’s comments about the brackets being part of the bumpers and the bumpers having to be entire within the bumper zone are a pretty straightforward reading of the bumper rules. If you design within those limits you likely won’t have any problems… however as some of the comments on this thread have indicated, interpretations aren’t always so straightforward.
If you ever have any questions about rule interpretation and want to be sure that your design meets the rules, use the Q&A forum. You have to think about how to word your question in a “general sense” as the Q&A cannot reply to a specific design, merely to the interpretation of a rule. In this case your question is likely along the lines of “Is the rigid fastening system referred to in R408.G allowed to extend above the top edge of the wood bumper backing material? If so, may the rigid fastening system extend beyond the 7 1/2” height limit of the bumper zone?"
I suspect you’ll get a “Yes” to the first part and a “No” to the second… but whatever the answer is it will be valid at every tournament you attend. And you’ll probably also get the qualifier that the answer is valid only “so long as all other applicable rules are followed”.
Then make it easy for your inspector by bringing a printed copy of the Q&A ruling along with you to your events so we don’t have to go and look it up… while the LRI’s do work to keep up with relevant Q&A decisions, and I always have a printed (and digital) copy of the Q&A’s available for my inspectors, many of them are new to inspecting and sometimes new to FIRST. Showing up well-prepared for inspection gets everyone’s tournament off to a great start!!!
Jason
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I’d love to see a Q&A on this sort of attachment next year. I know in the past (This was way back at champs in St. Louis) there was a team that had single-piece bumpers, designed to drop down over the top of the robot. The gap in the middle (for an intake) had some simple sheet metal spanning it, right up against their frame, with bolts - similar to yours, but attaching the two bumper ends and much wider. I know back then that was a problem because that section between the bumper segments didn’t meet the vertical cross section (figure 9-7’s equivalent that year) and that sheet metal was outside the frame perimeter without being a “minor protrusion”. Personally, I’d love to see this sort of attachment method normalized, I think it would make things easier for a lot of teams.
Our mounting plates are actually inside frame perimeter, the only protrusion is the bolt heads.