Sadly, I forgot to take any pictures while installing (in fact, the picture of the intercom I used here is borrowed from Google, rather than the actual intercom I installed this in). But this all came together very well.
There is a little bluetooth demod noise that leaks in to the amplifier, but it's a very small amount. The amplifier is overpowered for what it's doing; I think 20x would have worked out just fine. Other than these two tiny details, this is a success.
The amp's line out was soldered right to the inside of the RCA jack; the bluetooth receiver plugs in to an 1/8" cable to which I attached a quick and dirty ~13kHz first order low-pass filter (hidden in the 1/8" jack itself); the bluetooth power comes off of the full wave bridge rectifier in the amp, which is powered by the center-tapped transformer feed at 10v AC (which turns out to be 13v AC).
Having used shielded headphone cable for the audio lines, it was easy to locate the bluetooth receiver outside of the metal shielding and bracket of the intercom. Its range is pretty tremendous, and my friend reports that (one week in) they're happily using the bluetooth audio. And learning all about the ways in which bluetooth audio sucks, but that's a totally different problem for another day...