I've been working with the JP1 tools and have had some success. I added a RCA DVD to my Millenium 4 remotes.
However. I ran into difficulty when trying to program my Atlas DVR remote. Downloads were OK although I got an error message about conflict with fixed data at $17. Unfortunately, though, the remote would reject any upload. I identified the cause: the rdf file (SA_7SA_7) has some bad information. For checksum it reads ^$000:$002..$3FE. When I changed that line to ^$000:$002..$3FC, upgrades uploaded just fine and I could add an upgrades device code set for my Panasonic HDTV (LCD projection).
To fix the download conflict problem, I also changed the line $17=$09 to $17=$00