Yeah, I think I'll work on some sort of random pseudonatural ability generator that pseudonatural creatures would use in place of the -1 to attacks against them. The -1 to attacks would be included in the generator, but it'd just be one of the possible pseudonatural abilities the creature has when it assumes its alternate (native) form. It would make more sense for this roll to be made once, when the pseudonatural creature is first created (on paper), but the idea of a creature that has a different ability each time it transforms is appealing, too...
I'll post what I come up with when I'm not too tired to think.