There are obviously workarounds for this, but I'm wondering why the following query shouldn't work. It seems like it should. With MVCC already present on the back-end, I can't see any reason other than additional parsing routines that this couldn't work:
Anyway, I'm sure there are more important features for Postgres (like upserts, unique indexes on GIN, Gist and hash, fixed arrays, compact storage of enum arrays as bitfields, etc.) I just thought it was an interesting idea.