Well I'm not a big fan of this but in a lot of hentai with tentacles there's cum inflation.
So it could be a special button after climax that appears 2 seconds and if you click on it you can "burst" a big load.
Also be a little more complex to just be a simple cum inflation, randomly a girl will vomit and if there's other girls around they will be scared so you don't want to burst a load in the middle.
And IDK if it's possible but doing 1 or 2 special girls, you can fill them more and they will stay a little time knocked out and overly inflated with cum leaking in front of the locker.
Finally if girls pass they will be afraid and you will be unable to use the locker until she wakes up and run out, so better doing it in a strategic point where you can catch girls from right or left.
Edit : You could also re-capture a girl and fill her again to have the big cum inflation.