Why not set the program so that it accepts any note on, regardless of whether there has been a note off? it would also make playing rhythms with multiple fingers much more accessible.
The only reason I could think of for why this hasn't been done is to stop people playing multiple rhythmic notes on the exact same beat. If that is the case, then just make all the extra notes crosses. I'd much prefer that to this note off issue.

All the Best :music: