Am also currently dressed in the BMS polyester, just a really nice treble even compared to the Maggie 20.7s. Just sounds completely right to me and I did consider getting in the Fostex T-500 also.
Might still explore a second mid with separate tweeter for the Pap horn as a second option as the Pap is modular and allows for reconfiguration... a bit like DIY but with a stream of options. Am also considering the Autotech Iwata horn for the mid and tweeter.
That said I’m really enjoying the BMS and it’s making upgrading a bit of a hard choice as it does just work so beautifully in the Pap horn... I also have an upgraded cap with duelund bypass cap and path resistors on the way for the BMS compression horn so will be very interested to see where that takes it.
You could try the following 1.4" drivers: Beyma CP755Ti or Nd, Radian 745pb or 951Neo. I think most of these are lower in sensitivity than the BMS so a crossover adjustment would be required to match the level correctly. Not saying you should but perhaps as an experiment...