As a general rule, I'm just going with any plant that's been able to withstand my girls, because if they can hold up to them, then a rat snake won't be an issue for it... Which for the most part is just moss on the floor of their terrarium XD
I'm going to try and plant some lucky bamboo in there (the kind that swirls at the top) and cut the top off about a foot below the lid (they stop growing taller if you cut the tops. They are poisonous to ingest though, which is why I haven't tried it with the girls, but there's no sap or thorns that'll do any damage to the snake). I'll have a couple growing together and hope they stay standing.
I might also/instead do a small ficus, and plant it outside in my girls' outdoor enclosure if it ever grew too tall (they need it too. It's just a bunch of wooden boxes and rope out there, nothing natural for them to climb). I'm really undecided on the stocking of plants, and I probably will be until the last second. I have the look I want in my head, but finding the plants that'll match it is a whole other ball game. I might actually post a rough sketch to see if anyone can help, that is if you can withstand my grotesque art skills
I don't think the problem with the rat will be it trampling stuff rather than it just knocking it over trying to climb it (and correct me if I'm wrong, because I have no idea). I mean they don't have a huge amount of girth or weight to em, they just are zippy do-dahing all over the place and knocking stuff around and maybe crushing some of the more fragile species reserved for frogs and geckos, but I don't see my pothos losing more than a couple leaves with that snake.