For the record, I recently added hooks for mapping the paths in the LSCP
files for LV2 support which can be used to get relative paths. If
you save an Ardour session with an LS LV2 in it you can see this in
action (relative paths to symbolic links in that case).
I am not sure how easily applicable this is to other scenarios, but it
seemed worth mentioning.
 Which still needs multi-file gig and sfz support, I know, I know...