I think the bright orange RSCA OFF in the gauge cluster might alert you to perhaps turn them back on if the mod is done.
Sorry, I think didn't clearly type out what I originally meant. What I meant was your idea to install a full-time RSCA shut-off switch is great; and that I am the type of person who would remember to deactivate it once, and forget to deactivate it every single time I got back in the car.
Anyway, I did some wiring schematic reading and tested it out today...I think it's not going to be as simple as just installing an ON-OFF switch. The way the deactivation mechanism works is the SRS ECU needs to detect a ground pulse for at least 3-5 seconds
after you turn the ignition key to ON.
So basically, if you applied a constant ground to the input and
then turned on the ignition key the SRS ECU would never see this pulse; thus never deactivate the RSCA.
Additionally, it seems like keeping a constant ground applied to the input after it's deactivated is okay; but keep in mind that the OEM button is just a momentary switch that temporarily connects to ground when pressed. I did sit there and held it down for about 1-2 minutes while driving around and the RSCA remained off, but I cannot guarantee that it would not trip some kind of error eventually if you kept the ground applied indefinitely.
Finally, for the bit of good news, if you still want to try to install a manual cut off switch, I've come up with a way that most likely would work and is not too difficult. I've tested this out and it should work, but am not about to hack into my wiring to verify.
What you would do is install an ON-OFF switch directly connected to ground like you originally were going to do, but install inline relay which is triggered by the starter power. Specifically, you would want to hook it up so that the relay remains normally closed (and keeps the ground applied), and only opens temporarily when starter power is applied. This basically mimics the pulsing I described earlier, so that the SRS ECU can sense this and deactivate the RSCA.
Cliff Notes: You can install a manual RSCA shut-off switch, but it's a bit more involved. I can draw up schematics if anyone is interested.
Disclaimer: This is only theoretical and I am not responsible for whatever happens to you or your ride should you choose to try this 