As most development and testing (on Linux side) is happening in Ubuntu I feel the practical solution would be to emulate the Ubuntu install under /opt. This has been done on https://aur.archlinux.org/packages/freecad-git/
and seams to work nicely. Not the cleanest way to do it in our distributions but at least plugins seams to be happier in general.
I do not possess the knowledge to fix this in a way that would be generic and general for all Linux distributions, but if you do I would be happy to be of any assistance.
My attempts to fix this earlier revealed that I did not have the complete overview on all ways thees folders was accessed, and as @werner points out in one of the threads concerning this, is that it was never a prioritized task by any developer to have this working in any general way.
If you can do a better job than me, and provide a patch that would solve this for all distros i am sure your patches will be accepted and merged by the main developers.