I understand that.  Perhaps instead of moving that much work up the revlog could
be given an external data handler when you asked it for data.  And for the write
side you could give it an optional argument with the data for revlog to use as
the base for whatever diffing algorithm it might choose to use.

The contract would be that the external data handler you passed on read would be
able to retrieve that base for any revision in which you passed on such a base
on write.