The way I understand it the HEPAX RAM pages still need the HEPAX module itself. As with the Extended Functions Module, you can create 'files' in the RAM: data, text or program files. BUT here, when you store program files, they become part of CAT 2 and can be executed, your own on-the-fly FOCAL ROM if you want. Global labels are turned into XROM functions.
The drawback is that if for some reason, you add or delete a function 'in the middle', all other functions shift up a place - XROM calls are by number, not by alpha label. So make sure to put the most 'finished' programs first
You can somehow reserve a page to act as a ROM module as well, in which you can copy another ROM (or use the hex editor to create your own..). But even then, the HEPAX will need to remain plugged in, as that is where the page resides. I'm still in the learning phase, though.
The HEPAX is the Holy Grail of 41 modules, and its availability for the DM41X was, for me, the ultimate reason to get one. It is underway
In the meantime, I'll see if it works with I41CX+ on my phone