Hi Nigel,
It could be a long shot, but if ModeSMixer doesn't recognise the output format of your PlaneFinder receiver maybe you could try using Virtual Radar Server to do the job?
It has an option on the RX side that is specifically for Plane Finder, see attached VRS_RX.jpg.
If you can get it receiving data then there is a rebroadcast option you can select to be in BaseStation format, see attached VRS_Rebroadcast.jpg.
A bit convoluted I know, and obviously the settings I have aren't going to be much use to you so ignore them, but it might be worth a play around to see if you can get it to work.
If you click in any box there is a hint displayed at the bottom of the window as to what the field does.
You get to the config window though the Tools | Options menu.
Cheers,
Keef.