Have you considered Philodryas baroni? They are becoming more available in captivity as this species is very easy to maintain and breed. Some species in the genus Philodryas have caused fatalities, however P. baroni is very reluctant to bite and will just try to get away when threatened. Like with any other opisthoglyphous snake, I would use the same amount of caution when keeping and handling them. Philodryas.com has a lot of great information on the genus.
Hydrodynastes gigas is a great species as well, but I feel are a lot more work in regards to husbandry. I use caution with my male because his feeding response is unlike any other snake I have seen and he is very, very fast. I have seen tame individuals, but personally I would treat this species with more respect than some of the other mellower opisthoglyphous snakes you mentioned. Good luck!

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"El Respeto al Derecho Ajeno es la Paz" - Benito Juarez, 15 de julio de 1867