There's actually another use case here that's potentially extremely valuable for warehousing and other "big data": compact representation of a default value.
I too would benefit from tables having either a default value in the event of a NOT-NULL column being flagged null, or a flat-out constant.
This would be a big win in partitioned tables where the partition can only hold one value of the partitioning column.
I guess a constant would be a pg_type where the sole value is encoded, and the column itself is stored like an empty string.