XMP to RawTherapee (pp3) converter?

This is a feature that I would also like, and I would argue that it's not unprecedented, considering that there is already support for RPPS files, however I doubt the devs would ever add this since its too niche. For my I would be happy with just having the "Star" ratings implemented. Things like exposure compensation might not work very well since FRV defineately is applying a baseline tone curve to the image, presumeably the same one as Adobe, but I guess it would be possible to simply apply that same tone curve to all your images in RawTherapee on import.
I'm sure you saw my discussion on the RawTherapee forums about my workflow where I first cull my native ARW files in FRV, generating all the nice sidecars, then I convert to DNG files (NOT liner DNG, just the regular DNG so as to preserve all the raw bayer data). Adobe DNG converter detects the sidecar XMP and embeds it into the output DNG files, so then RawTherapee picks up the XMP metadata properly.

Honestly, I would say the burden here should lay on RawTherapee and not the devs at FRV. XMP is a standard format, and its RT that should better support it, not the FRV devs adding support for such a neiche format.
What I would be happy with would be a setting to allow FRV write to XMP embeded within DNG files. I konw FRV has said before that storage media is not stable enough to safely write to embeded XMP in DNG, however they recognize themselves that metadata is a mess, and from my experience, writing sidecar XMP while there is already embeded XMP is just a recepie for disaster. Embeded XMP seems to be the way Adobe wants and expects XMP to be used with DNG files, and organizations such as the Metadata Working Group recomend that XMP be embeded into DNG files.

With the understanding that FRV is concerned with storage media stability, I would propose that a very usefull feature would be to give the user an option to use embeded XMP in DNG files, possibly as a "hidden" setting that isn't actually visible or easily acessable in the menu, such as in a config file that needs to be manually edited. Only for users who REALLY know what they are doing, and have taken all the proper precautions with backups etc.