Rob,
AlanG used to use a similar setup, though he now connects the Bluetooth 'receiver' into a dedicated auxiliary input on his intercom.
I have also tested a similar setup, but currently have no problems with a hard wired cable connection direct from the PAW to an auxiliary audio input on my (Flycom) intercom.
One problem you might experience going in through the 'back door' is that the audio level could be too low. Although I've never had a problem, several users in the early days of audio alerts had to introduce small amplifiers to boost the relatively low audio level from the PAW.
Regards
Peter