Out of IAD, you really have little choice. You can possibly move your business to DL, CO, AA but these options will force you at least one extra connection, which can be aweful during irregular operations. I think there are no airline programs that provide you free lounge access to their mid-tier level elites (I can't think of any that does even to their top tier elites.) DL used to give complimentary Crown Club membership to their top tier Plat Medallion, but they cancelled that last year, I think.

As for other airline lounges, I heard that AA, CO, and DL lounges are somewhat better than RCCs, but not too far ahead of the game. If I were you I would just stay with UA, despite the fact that you are not so happy with them. If you must switch, choose one of AA, CO, DL based on your flight pattern and hub connection. AA (DFW, ORD), CO (EWR, Houston), DL (ATL).

Good luck.