Using the obsolete extension for reimporting commits
The obsolete and the evolve extension from marmoute might provide the tools to make it possible to turn hgsubversion into a real (mostly) transparent bridge.
It would allow marking revisions which were reimported from subversion as obsolete while adding a pointer from them to now revisions. Changes which decend from the now-obsolete revisions can then be evolved automatically to be based on the new revisions instead.
The only missing piece to transparent operations would be merge commits. If graft commits can be represented in SVN, a merge could be represented as a set of grafts with an (optional) additional commit after calling hg revert -r <original merge commit> --all to include all manual merge fixes.
Do you think it would be possible to use obsolete and evolve once they get integrated into Mercurial?