BACnet stack for Linux

OK, understood - thanks.

Is the write stuff hard to do (if, for sake or argument, I had a spare BMS system that could cope with being broken)?

The job I need to do requires setpoint writes so BACnet4linux would be out right now, but I'd like to put some effort into it and see what I can do, as I'm otherwise faced with some fairly expensive, inflexible and over-the-top solutions (usually Win* based) as alternatives.