Since most RGB kits are 12V anyway (like the Asus AURA "standard"), wouldn't it suffice to just seperately power the 12V and let your button control the RGB channels ?
Or maybe use your controller PCB seperately for use atop a USB header, you could possibly piggyback of the already done design and production to expand to a whole new market in dire need of RGB controlling with lightweight software and tiny hardware.
See my reply here. That got quite long.