#define MAX_AUTO_IT_CHARS (64*1024)
...

static char g_chAutoITbuf[MAX_AUTO_IT_CHARS];   // 64k buffer

// this works fine & returns the expected result - namely
// "Testing ability to return a string to AutoIT from C++"
CHAR *OMyClass::SimpleCharTest()
{
    _snprintf(g_chAutoITbuf,
              MAX_AUTO_IT_CHARS,
              "Testing ability to return a string to AutoIT from C++");
     return g_chAutoITbuf;
}

// This does not generate any errors in the Auto-IT Script.
// However it does not return the proper result.
// And, if you run the Debug Version, you will get ASSERTS in
// AfxTempl.h  and AfxWin1.inl
//
CHAR *OMyClass::ComplexCharTest()
{
	// This is where I need the code to go out and do a bunch of stuff
	// and place the results into a string
	// If I call this method from the EXE directly - it works as expected
	// If I call it from DllCall() via AutoIT, it doesn't work
    CString str = DoMFCandATLStuffHere();

    _snprintf(g_chAutoITbuf, AUTO_IT_MAX_CHAR_RETURN_SZ, "%s", str);

    return g_chAutoITbuf;
}