I was checking out the OSC functions from SongMaster Pro, and noticing that it sends /play (lowercase) for its transport control. I also checked with Reaper, and it also sends /play (lowercase) for transport control (which can be changed in their confg file). Looking around the 'net, it seems most references to OSC ‘play’ transport control use the lowercase /play. Since OSC addresses are case-sensitive, that makes the GP /Play (uppercase) command an outlier and incompatible without scripting.
Any chance you can include lowercase /play as a recognized command?