This is not lesser known among JTACs and ACE, as you use a crap ton of radios and do this already (I think).
In ITB, we taught Acre 'transmit' setup on three different keys (Capslock, Ctrl+Capslock, etc), mirroring the old TFAR setup. We did this to make it easier for new GCE joins to get up and running on two radios (Team and Squad nets).
I recently switched to using one key to transmit over the currently active radio (Capslock) and one key to cycle between active radios (Shift+Capslock).
I do recommend it as you start to use more than 2 active radios at a time. I haven't tried it on vehicle racks but I assume after you've added the vehicle rack to your active radios (via ACE interaction menu), you can incorporate it into your cycled radios.
You can also use Ctrl+Up/Down arrow (or set it to a different keybind) to change channels on your currently active radio, without having to open that radio. That should make quick work of setting up vehicle radios.