[AndyD]

Не уверен, но попробуйте так

ComDispFunction funcGet;
    COMVariant varRet1;
    COMVariant varRet2;
...
    funcGet = new ComDispFunction(com, "LinkTest", COMDispContext::Method);
    varRet1  = new COMVariant(COMVariantInOut::OUT_RETVAL, COMVariantType::VT_BSTR);
    varRet2  = new COMVariant(COMVariantInOut::OUT_RETVAL, COMVariantType::VT_INT);
    funcGet.call(varRet1, varRet2);
    data = varRet.bStr();