I'm looking for an AR-308 barrel that is cut with the 25 degree taper, to match up with direct thread suppressors by SIG or CGS.  Examples of factory rifles with this barrel include the Q Fix, SIG Cross, and SIG 716i.  I know that ADCO Firearms has a service for "Contour SIG Reverse Taper", but they will only do this if they are also threading the barrel at the same time, which would mean I would need to find a non-threaded barrel to send to them.

Does anyone have any recommendations for an aftermarket barrel to fit a DPMS-pattern AR-308 that has this 25 degree taper?

No lead on the barrel, but I would ask Class 3 Machining if they could cut the taper on a existing threaded barrel. They have a dealer account here and posts in the EE.
Class 3 Machining

Thanks!  I contacted them, and they said that they can add the taper without having to shorten the barrel to re-thread.  Great find!

Yeah, I had mine done by local smith who did no need to rethread it