I've been messing around with task_backass.dat just for fun, and have found that you can change loc.state.anus_state to open and loc.state.anus_state_v to a value greater than 0 (be careful with this one! I've set it to 0.02.) and she'll gape until another command causes it to reset. Unfortunately, it's a drastic change after you pull out, her anus reopens. I'm not sure if there's a way to just make it stay open or not.

Also, if you look at char_base.dat, there's a bunch of values you can tweak in there related to the starting appearance of the girls.

nice