Basically, FIRST is going to use common sense for this. And it will be up to the inspectors when they are inspecting in the pit area and up to the referee from match to match. (Still waiting to be briefed before the regional competition for guide-lines when doing inspections as an inspector. Donˇ¦t worry, I will be as tough as any inspector can get.

)
As soon as they see that part of your robot extending out have a possibly of getting stuck, they will call it illegal. So, avoid using anything long and flexible, or sticky, or have risks of getting caught in sharp edges. You have to imagine all sort of possible situation for your device to be entangled in.
I think it will end up a match to match decision, since most teams will try really hard to make their devices not likely to create entanglement.
So make sure you use something solid and strong. A bunch of telescoping tubing should be safe... a triple-folded should be too...
An arm with Velcro or tapes (sticky side out) or little hooks probably isn't legal. If you have little rubber band sticking out, even just one, itˇ¦s probably at a risk of entanglementˇK You get the idea.
I have a feeling that this ruling will be different from place to place. Especially if the device is right in the middle between legal and illegal, and the inspector is having a hard time. Some inspectors will be easy going and let the robot off the hook... some inspectors won't be as lenient. I believe a lot of robots will past inspection, and will be left to the referee to make the call.
Considering the number of matches, the intensity of matches, and the randomness of what teams will be doing, I really think that the extension will face plenty of challenges during qualifying rounds to determine if it is legal or not. So, make sure ALL parts of your robot is robust enough, including something really light that will extend with 8 seconds left in the match about 20 feet, with no other robot to bother it.
And make sure your extension DOESNˇ¦T fall out when you donˇ¦t want it to. Have it lock in place really hard before you want it to extend out.
By the way, I think a lot of teams will get away by only releasing the extension the last 7~8 seconds. The less the referee see the device extended, the less likely it will be called on, even though FIRST say they will treat the extension as being released at ANYTIME during a match.