Pete,
I have some good news: I could compile ModuleUser64.c included in my project, so it could link without ModuleUser64.lib and without any problem.
This time I am 100% sure, I have a 64-bit DLL generated. Unfortunately I cannot test it because I don't have P3D v4 yet. I will send it to a friend who has it and ask him for feedback. If you want, I can also send it to you, it is part of my External EasyFMC product (for more information: http://emarciano.free.fr/ExtEZFMC).
Otherwise I can send you the testing gauge I promised, if you can wait one more day :)
We are almost there, I am sure it is going to work soon.
Eric