I haven't tried yet, but I assume you need low respect to become Galiano's b*tch, then wear a faulse or hacked slave's collar to look like an upper citizen's slave ?
Viewing post in Lust Odyssey (NSFW Visual Novel) comments
That's the problem with the respect scale, you can't just choose your dom/sub role on a simple dialogue choice, and need to have a high/low rank of respect to unlock such behaviors.
So, try to lock increase respect in your diary, so that it only decreases, then behave as a big sl*t, and talk to Galiano from time to time to feel the temperature. Also, make sure, still in your diary, that your not an exclusive dom (behave as a vers or sub).
For example when working at the bar and servicing the gang, accept your fate to be hazed. It could require numerous days, so, fight during the day (the thugs for example) to gather some XP, or go to dark alley at the end of the night and wait for what's his name again, then go to the club and work.