Since I tried these function modules some years ago dealing with a similiar requirement I know how much effort you probably spent by trying and debugging to find out the way hou to use these SAP internal and not documented function modules of group COXT.
Although you are deservedly happy now because it is working you should not forget one thing which finally stopped me in the past using it.
These function modules are not released by SAP. This means they can be changed with every new release, enhancement or support package. SAP does not guarantee to keep parameters and functionallity. From this point of view they are not better then using CALL TRANSACTION.
I know that it is practise to use SAP internal function modules in own programs and I do not condemn it but I recommend to consider it at least. Some companies have got policies not to use unreleased function modules. Again not released in this context means not released for usage in customer programs.
You can see the status within the attributes tab. Compare it with any of a released FM like CONVERSION_EXIT_ALPHA_INPUT. There you'll find a Released on date.
/Michael