Hello Chris,
I wanted to offer a suggestion that would improve the time it takes to switch versions of mpd. It seems like you could provide a /opt/mpd/"version"/ directory structure where you would compile each supported MPD version to for a given release. This way either a script that starts mpd or better yet the service file would only need to be updated when changing versions. I don't believe MPD itself has any dependencies yet, but if there was ever a conflict with other packages, for example mpc or even the mpd.conf file, this would also provide an easy way to address that.
The only reason I bring this up is that it seems that the selected MPD version is now installed at every boot. I also noticed the installing message doesn't update once it's installed, or perhaps it updates on a timer vs using a dependency.
Thanks,
Jim