What dangers are there putting power through RCA connectors?
In my experience RCA plugs make a very tight connection, and I expect the resistance to be way less than 50 m&Omega;, probably near 10 m&Omega;. Even at 2 times 50 m&Omega; (you have to count both pin and sleeve) a 500 mA current will cause a 25 mW dissipation, which is more than acceptable.  

Matt makes a valid point about mixing them up with audio plugs, though. The wall-wart probably has a DC jack like this  



Why not use a socket for that? These are almost exclusively used for power supplies, so there's little chance of confusing them for something else. Sockets for them are readily available.