We are working with postprocessing from VC to UR.
Does UR have or plan to issue official documentation for generation of URP files?
No, URScript documentation is public, script can be included from URP program.
The format of .urp file is not documented anywhere and to my knowledge there is no plan to do so (there is just not enough demand to justify the effort). You can open a .urp file with some tool like WinRar and unarchive it and view/edit the ascii version of the file. It is basically a proprietary UR format buried within XML.
I know other OLP companies have reverse engineered the format but it does change from time to time.