I'm just finished and going to sleep, so a few words only.
This is alpha release, just for testing. Report bugs please.

That version limits:

0. I've not noticed the NSIS team inverted the 'calling convention', arguments
are passing to stack from left to right during d::p call, so you should invert
system.txt arguments order. See SystemEx.nsi for examples.

1. PT_VOID and PT_BOOLEAN are unsupported.
2. PT_LONG without Pointer specifier is not allowed. 
3. NSIS->Call PT_LONG converts as 32 bits. Back conversion bit-width is unknown.
By the way NSIS doesn't support int64 operations, so you can process received 
long values as strings only...
4. Generaly untested :)
5. Requires MSVC++ 7.0 to compile :) Sorry....

Todo (will be done in a few days):
1. Structures work (thru functions).
2. Callbacks.
3. Arg-lists
4. Cdecl convention
5. Optimizations and so on...