I found a LMS plugin that executes scripts on the LMS server based on player status/events, squeezedvera.
So I "press" the power button on the squeezebox app and squezzedvera executes a shell script in LMS that opens a CGI page (wget) in the RPI, that executes a python script and I have GPIO control!
So now I just need to get and wire the relays.
There should be a lot of cleaner options, but this seems to work nicely, I have only tested with a DMM. Off 0v, On 3.3v.