Access to non-UA domestic lounges is not a rumor and is a clearly stated policy. FTers have long been making use of this stated policy and there are multiple existing threads on accessing domestic *A lounges.
Access to LH Lounge at EWR includes the SAS lounge
Access to Lufthansa lounge in Boston as a domestic itinerary *G
Consolidated "Access to Lufthansa Lounge at IAD" Thread [Merged]
Access / reviews for TK's IAD *A lounge
Best IAD Lounge
Lufthansa Lounge Access in DTW/Detroit
Better lounge in LGA: United Club vs Maple Leaf?
UA domestic itin and LGA AC MLL ?
International Chicago*G Lounge includes LX & SAS
UA/*A gates & lounge situation at MIA
Maple Leaf club access at LAX
Using AC MLL in the US?
I think this is most of the *A *G lounges presently in operation (there are threads on others but now closed such as the SFO BR/SQ lounges)
Essentially if you can get to the lounge and are UA *G, you have access unless closed due to crowding (SQ & BR in SFO did this regularly). Some of the *A lounges are pretty good and a few are not really worth the effort.
BTW, this access also apllies for UC members,
Does United Club Membership Give Access to * Gold Lounges in the USA?