Regarding the LR processing tags, those are now in exiv2: http://dev.exiv2.org/issues/1190
Getting back to the original question, we bumped up against RT’s unusual way of handling XMP data a year ago. This is issue #2307 in RT: XMP & IPTC metadata missing or corrupt · Issue #2307 · Beep6581/RawTherapee · GitHub
We are looking at workarounds in exiv2 so that other applications (digikam, shotwell, gimp, etc) can use the XMP data that RT writes in a non-standard and non-spec way. Comments and/or patches are most welcome! http://dev.exiv2.org/issues/1154