Hello, just stumbled upon this thread, and registered just to get a reply in :D
First, thank you for the great plane, it's really enjoyable, honestly, biggest downside for me is the prop buzz from the default MSFS TBM 930. It would be great if any future update could change the sound, even if it wouldn't be exactly TBM 850 sound.
Anyway, back to topic, I believe KAS 297B is used in the PC-12/47, and while the SWS model has it's own set of problems in my opinion, I believe the altitude and VS selector is working correctly, including arming the altitude and engaging the VS mode. Their PC-12 also supports WT GNS530, PMS 750 and TDS GTNxi.
I'm not sure if it would be acceptable for you (or them), but maybe they could point you in the right direction of how they implemented it.
Anyway, thanks again for the great mod, looking forward for more to come.
Best regards.