I don’t see a very big value in the proposed extensions in general. But I agree there might be cases when it helps.
It would be nice to have a connection between the roslaunch machine tag and the compute resources in URDF (not sure how RO2 handles remote machines, though).
From time to time, there are other similar requests to extend URDF. What seems best to me would be to just define some optional extensions and a way to parse them. I think the newer SDF versions add support for something similar. However, to be able to define extensions, you first have to have a schema of the base. That has long not existed, or at least not in a canonical form. For long, the “definition” or URDF was the urdfdom parser. I think last year somebody took this issue and created a repo for the XSD schema of URDF which should one time become the canonical definition of the format. But my google-fu failed finding it…
Here are some resources I consider valuable to read if you’re interested in this topic: