Index only scan paving the way for "auto" clustered tables?

Hi all,

I wonder, could the recent work on index only scans pave the way for auto clustered tables?  Consider a wide, mostly
inserttable with some subset of columns that I'd like to cluster on.  I'm after locality of tuples that are very
frequentlyfetched together, but not keen on the downtime for a cluster, nor the maintenance that it requires.  Would it
bea stretch to have an index that branches on the subset of "cluster" columns, but still stores all the columns, making
ita covering index?  Given that we can already index concurrently, such an index would not require downtime, and would
beself maintaining.  From my understanding of the index-only scan implementation, I suspect that such an index would
effectivelygive locality, with some caveats…  

I'd expect the overhead of inserting in to such a table would be high, perhaps prohibitive.  Perhaps better ways have
beendiscussed.  Stupid idea? 

--Royce