Sena seems a bit confused on the ability to actually pair GMRS radios with their own set. Seems that the user manual says you can,
but Sena support says you can't.
This could be exclusive to the 20S.
But aren't GMRS radios coming with Bluetooth now? Couldn't that allow you to pair to the Sena?
Sena makes the SR10, which is a BT adapter you plug into the GMRS (or other type of longer range 2-way radio). Pair your helmet headset to the SR10, and mount the PTT switch somewhere convenient.
I used to use a 440MHz ham handy-talkie w/vox, wired to a J&M helmet headset. I don't think the simplex vs full-duplex issue is a problem at all.
As to the legality issue - the Sena itself isn't legal everywhere. Some states don't care, some require one ear free, others you theoretically can't use them at all. I figure it's kind of like earplugs - I can ride if I'm deaf, but I'd rather take my chances with an earplug ticket.
From 20S manual:
"• In some regions, it is prohibited by law to ride motorcycles while
wearing headsets or earbuds. Therefore be certain to comply with all
relevant laws in the region before using the product."