David, I too built this mjoy16 (along time ago) and still use it today. In fact I have just added another 8 switches last night.
The only way around this limitation is to use the program "mjoy mapper" that Mindaugas wrote. Its available from his site, or you can email me and I'll send it to you. There are probably other joy to key softwares around, but this one was designed especially for MJOY16. In fact I helped him with the testing of it. If anyone is in contact with MeanDog, I would like to chat to him again, as its been a long time since we caught up.
But, it would be great to use all buttons directly to FSUIPC, as it would be much simpler to setup. Alas, MS dont like more than 32 buttons, so we have to live with this method.
Jon