God, its been ages since I've even thought about PBMs... but they were a great diversion as our group's interest in D&D faded over the years.
I was a big fan of The Kings of Steel and Saturnalia (?), don't suppose anyone has the Kings of Steel map or Rule book to hand - its was a simple yet satisfying game that may translate well to a simple turn based game/app... ;)
Keep the spirit alive!