I haven't spotted something similar to the yankey hill flip up front that would go on the light weight barrel, so would it work if I made a collar or bushing to fit the barrel for the ID and the block for the OD?  I figure the faintest interference fit for the collar and make the gas hole a bit larger than what is needed.  Maybe a hundred thousandths oversized, that way alignment wouldn't have to be dead nuts on it.  Is this asking for trouble?

www.pri-mounts.com/mm5/merchant.mvc?Screen=PROD&Store_Code=PRI&Product_Code=05_0026&Category_Code=AR15_M16_ACC
PRI makes what you are looking for (I think)...

Also,
armsmounts.com/catalog.php?action=124&item_id=104
ARMS was supposedly going to make a version of the Silouette for .625" diameter barrels.  Not out yet AFAIK...

The PRI unit is what I am looking for, costly, but what I am hunting.

I believe that YHM either makes, or used to make a lightweight profile version of their flip up...  I'd give 'em a call.

- AG