It's an attractive idea, and anything is possible if you want it badly enough.
IMO, it's more likely to happen at the OEM level than in a home workshop. It's an engineering job, rather than a simple woodworking task, and more complicated than dropping a preamp into a passive bass. Things to consider:
Most transmitters are moulded in plastic or metal, and not made in such a way that switches, LCD displays, etc are easily convertible for installation in a bass.
The control cavity has to be considerably enlarged.
There are a number of parts that need to be accessible from outside the instrument, depending on the brand:
- infrared port for pairing frequencies to the base unit, at least with some brands
- antenna (the pcb cavity needs to be shielded, the antenna needs to be in free air and out of the way)
- charging jack (or battery dock, in the case of the Shure GLXD, with its interchangeable battery pack)
- display window and/or power/status LED
- power switch
- an emergency bypass jack for use in passive, wired mode
Other parts need to be at least accessible inside, like the sensitivity control and the battery box. Tool-less access would be preferable.
There are other factors:
- Expense. Most techs would charge at least 3 or 4 hours for a job of this scope. Also, you're buying a rugged housing, belt clip, miniature jack, etc., when all you really need is a tiny PCB and an antenna.
- Most basses last for decades, whereas wireless technologies are evolving, and the hardware changes every few years.
- Interchangeability is low in the wireless industry. If you install a G30 transmitter in your bass, what happens when you want to move up to a G50? Or a Shure/Sony/AKG/whatever?
- If your wireless fails, that bass is out of commission while you seek repairs.
- Extra transmitters are required for each bass, at a cost that's higher than some preamps (even before you account for the labour to butcher the bass and transmitter).
- Resale, especially if the buyer wants/needs a different brand or model of transmitter.