In a previous post regarding my experience playing a slave, I explained that you are meant to have something of a "bad time", a measure of catharsis brought on by a fictional negative experience. I also went on to explain that apart from number-grinding and having your character referred to as "SLAVE" in dialogue with folks, your experience rarely tends to differ from playing a free character in a socially-disadvantaged position. Of course, there are property restrictions and the ability to be summoned by your slave collar, but these consequences are brief and easily-circumvented.
Some slaves roleplay their situation with aplomb. They are downtrodden, they follow the restrictions placed on them by their master for fear of in-character reprisal. They do things they hate. But not all slaves do this. Many others sass drow matrons in the Hub, or walk up to fully-armoured orog commandos and ask their trade skills without so much as a bowed head. I can't count how many evil-worshipping elves I've seen in the Underdark who treat their slave collar as a shield (mind you I don't think elves should necessarily be taboo in Andunor, but that's the player culture), continuing to roleplay as they would without it except now, they're accepted in the UD. Still others continue to flaunt their level 30-ness for all it is worth; their characters are valuable resource-gathering tools that can remain connected to their masters and essentially, disregard potential character consequence.
Therefore, I propose a mechanical consequence: make the slave collar item apply a permanent level drain to level 20.
This would not prevent slaves from doing writs or gaining XP. With the actual mechanical level drain status effect, you can still gain XP, but per the NWN wiki: "The draining effect of each negative level applies a -1 modifier to attack rolls, skill checks, caster level, DC of spells, and saving throws." Also, if you are a commoner slave, you would still have all of your crafting points given by Craft Mastery once you progress past level 20.
I personally believe that this would preserve more of a harrowing, difficult atmosphere surrounding slavery, and make people think twice about taking the collar as a speedrun to hang out in the UD. Sure, you're now property and shielded from excessive bodily harm, but there are consequences to your decision. You are weaker. You have to team up with other slaves to get things done, and you can't just be throwing hands against your master if they do something you don't like. It means you have to really think about this very grave IC consequence and what it means for your character.
Also, just so we're on the same page, it's not the end of the world for me if this suggestion doesn't go through. I'm sure there are consequences to it I haven't thought of. In all honesty, I want to float it to see how the playerbase and developers would react.