I called a welding shop and asked if they could add bracing to the bumper mounts on our Rushmore so I could carry bikes. The man said to bring the trailer by and he would look at it, but in almost every case he has seen, the only safe solution was to weld a complete receiver hitch to the frame.
I took our Rushmore to him, and after looking over the mounts, he was very surprised at how "beefy" the installation was. Said he had never seen a trailer bumper that was made out of such heavy gauge steel, nor such strong welding of the bumper to the frame.
He said I was "good to go as is", however, if I want I could come back after carrying the bikes on a couple of trips, and he would inspect the welds - just in case.
Regarding voiding the warranty, I'm not concerned about that as it will expire soon anyway.