I've heard of a lot of kids and older people doing this - its as if the warm water relaxes their body and they just go!
All I can think of is to sit her on the potty next to the bath while you are running it. I use to do this with DD as she would always do a wee, it helped with the toilet familiarisation. Maybe she might just do a poo...
The only other thing is to change the time of the bath so that there is less chance, like just after she does a poo.
Best of luck.
DD is 3yr 8 months - DS is 6 months