Along with the blinking RSL light, can you plug in external LEDs (e.g. robot under glow) into the RSL slot on the roboRio to blink along with the signal light?
I would say no for two reasons, the first is that although there isn’t a specific rule against this there is a part number listed in the RSL rule(R70), and the second being that even if there wasn’t a rule against it I doubt the RSL port can deliver enough current to drive that many leds.
If you really wanted them to flash you could use a relay and flash them on and off or if they’re rgb leds use an ardunio or the RoboRio for signal and VRM or PDP for power
Interesting question.
Because LEDs are custom circuits, their connection must satisfy R60. One part of that rule is that power pathways to robot control system elements listed in R71 may not be altered. The RSL is listed in R71.
So the question is, does wiring LEDs to the RoboRIO’s RSP port alter the power pathway? I think it does, because current drawn by the LEDs reduces available current for the RSL.
It might be argued that an LED string that draws less current than a second RSL (allowed per R70) would not overload the RoboRIO’s RSL port; however, verifying that is not an inspection task, it is a design review.
The port is meant to supply a current of 120mA to the RSL and nothing more. If you really want it to blink along with the signal light, it’s best to do it via a Relay Module or a custom circuitry. I do not think you would want the LEDs to blink with the signal light if it was browning out though 
Since the robot rules R70 is under the title “8.8 Control, Command & Signals System” allows only one or two RSL indicators. I see that as allowing only those connections be made to the RoboRio output.
R70. ROBOTS must use at least one (1), but no more than two (2), diagnosticRobot Signal Lights (RSL) (P/N: 855PB-B12ME522).
Any RSL must be:
A. mounted on the ROBOT such that it is easily visible while standing three 3 ft. (~ 100 cm) in front of the ROBOT,
B. connected to the “RSL” supply terminals on the roboRIO,
C. wired for solid light operation, by placing a jumper between the “La” and “Lb” terminals on the light per Figure 8-12.
An outside-the-box suggestion:
Use a photosensor next to a second RSL, perhaps in a light-sealed box for protection against interference, to control the LEDs.
Tha would work, except for the “any RSL” part of the rules
Please clarify what rule you are referring to. I don’t see any rule that would disallow Alan’s idea.
This seems like an “inside the box” idea. 
Custom circuits can run in disable. An LED controlled by DIO can mimic the RSL. I would be concerned about anything plugged into the RSL other than the specified signal light.
R70. ROBOTS must use at least one (1), but no more than two (2), diagnosticRobot Signal Lights (RSL) (P/N: 855PB-B12ME522).
Any RSL must be:
A. mounted on the ROBOT such that it is easily visible while standing three 3 ft. (~ 100 cm) in front of the ROBOT,
This seems to imply that any RSL placed on the robot must be visible.
Of course, without the box, it’s perfectly fine.
Thanks, now I see it. ::rtm:: 