I'd really like to see us find a way to apply some version of this patch. I was in favor of the concept 3 years ago when we did this the first time, and I've subsequently done quite a bit of work (viz., MVCC catalog snapshots) to eliminate the main objection that was raised at that time. But it's really hard to reason about what might happen with lowered lock levels, and convince yourself that there's absolutely nothing that can ever go wrong. I don't know what to do about that tension, but I think even modest improvements in this area stand to benefit an awful lot of users.
Wouldnt MVCC's strictness rules pose harder restrictions on pg_dump instead of relaxing them?