Lol dude if you knew anything about programming you'd know it would take him less than 1 minute to implement this tops.
For your set up perhaps, but remember anything Steven implements has to work for all MusicBee users which is where the extra work comes from.
It would also require users to have tagged their multi-disc albums correctly.
Otherwise it becomes a point of confusion because it would function the same as the existing Play album only
Depending on which view you're using this can already be achieved by displaying a sub-heading based on the disc number and using that to initiate the playback.
You could also set up a custom tag for the extra tracks in case the album isn't a neat disc-by-disc split between album tracks and bonus tracks.
Bee excellent to each other...