@Black-Square I posted to the FSUIPC support forums, and John Dowson had this to say:
Some standard controls don't work when assigned externally, and this is aircraft-dependent and I assume an issue with the event implementation in the aircraft. I cannot do anything about this in FSUIPC. 
I did sort of see this experimentally while I was checking my technique for calling K events via FSUIPC; the Kollsman Increase K event works, the BATTERY1_SET K event works when given a state (1 or 0), but as an example I could not use K:MIXTURE1_SET with any value and get the mixture lever to move.  I don't think I had any success trying the LVar, either, but I also had a control bound to a hardware axis in the sim so that may have been interfering.  Left Pitot Heat (hence the above question about the existence of right pitot heat in the Bonanza) worked, as well as the Alternate Static Air K events worked, as well.
Nick, does what John said ring any bells with regards to how the magneto might be implemented compared to something that I could get working, like the Kollsman increase or Alternate Static Air?