The uMP3 documentation says that pin D can be use for a "hardware busy indicator". This is not the default but available by using setting ST H 1.
My question is, can this indicator be connected to a prop-1 PIN and used similarly as a trigger to indicate when the player has stopped? If so, what would that connection look like?
Falling into the "had it been a snake it would have bit you" category ( ;D ) I found this in the uMP3 docs:
The μMP3 can indicate when it is busy playing an MP3 file. You must use the Settings "Hardware Busy Indicator" command to enable the indicator. Once set, the pin labeled "D" is configured as an active output. When a file is playing, the indicator is high (5 Volts). When the system is idle (no file playing), the indicator is low (0 Volts). If a file is paused, the indicator will remain high. If the playback is stopped, the indicator will go low.
(my emphasis)
So long as you're powering the uMP3 from the Prop-1 you can bring the uMP3.D pin to any Px.W pin configured as an input. When the uMP3 is playing you'll see a "1" on that port, when it's stopped you'll see a "0." If you have a spare pin this is an easy way to check status -- if not, you can get that through the serial link.