Passing INTEGER*2 arguments using STDCALL convention fails

My initial test for this issue is as follows

C function...

#include <stdio.h>
extern 'C' void __stdcall PassIntegers(char i1Arg, short i2Arg, int i3Arg)
{
  printf('%d\\n', i1Arg);
  printf('%d\\n', i2Arg);
  printf('%d\\n', i3Arg);
}

Fortran program...

program test
 STDCALL PassIntegers 'PassIntegers'(VAL,VAL,VAL)
 integer*1 i1
 integer*2 i2
 integer*4 i3
 i1 = 7
 i2 = 45
 i3 = 19
 call PassIntegers(i1,i2,i3)
end program test

This works OK and I don't know how I can progress from here.

When you compile a function/subroutine using FTN95, arguments are always passed (in) by reference but you can pass arguments (out) by value from FTN95 when the function/subroutine is compiled using some other compiler such as SCC.

When attempting to pass an int2 (val, stdcall), does the ftn95 compiler simply promote this to an int4 and pass this instead?

Yes. If you look at the /explist produced by program test above, then this appears to be the case.