Since calling 32-bit dlls from the 64-bit world seems not to work, I tested IPC using RegisterWindowMessage and SendMessage. You can't use pointers, of course, but two dwords (or two REAL4) can in some cases be sufficient to tap a 32-bit library from a 64-bit app.
Results show that it's not a good solution for an innermost loop that runs a Million times: A single exchange of dwords takes roughly 10-30,000 cycles. Still, if some specific app ported from 32- to 64-bit depends on some exotic old 32-bit library, and speed is not an issue, it might be an option.
Attached three executables - extract to some folder and launch client.exe. The sources are also there, in an arc file with two PellesC projects and Client.asm (or Client.asc for those not scared of colours ;))
