Hmm, it could be possible to use the existing values. Rather than a meter, it would be something like, "Bladder status: Empty/Low/Medium/Bursting" or something like that
A possible location for it could be under where the info about your characters dirtyness and hornyness is. I don't have the option to open the game to check right now, but IIRC there's a section under the Status menu that shows how clean/dirty she is, and also how horny she is. IMO it would feel natural to include a bladder and lactation gauge there
Either way, just want to say that you're awesome for communicating so closely with your players like this! Even though I'm a brand new player and have barely gotten to chapter one, you're taking the time to read my comments and answer, it's very nice to see!