My mother had Stage 4 mucosal melanoma with the primary in the rectum, as you describe. There are only these experimental treatments available. She was on AZD 6??? but that did nothing, so she switched to Gleevec. That was also unsuccessful but you need the right gene mutation, which she did not have (we later discovered). She passed away within 4 1/2 months from her diagnosis.
She was at Mt. Sinai Miami Beach and Memorial Sloan Kettering in NYC. These experimental programs can be found nationwide in various settings. Another drug to look at is Sutent. There are several more and I haven't kept up with this since her passing in July.
Also, Dana Farber Cancer Center in Boston has a free service that helps you find experimental programs. I think the lady's name was Marianne. She is very helpful in finding Phase I, II and III experimental programs.
I wish you all the best. I wish that I had better news to tell you but don't give up hope.