... I have an idea that you can bash down immediately, if you with. I definitely want to be involved in this event (I have always loved Essyrn and find colosseums to be of interest), though I would have to make a new character no matter what.
Anyway, I wish to play an 'outside' role in this. Instead of being one of the slaves, what if my character was the daughter of the Shah herself? But unlike her tyrannic mother, she wants to help the slaves. She would provide a larger range of influence for the slaves (if they needed information to get outside of the colosseum—the slaves do live there, correct?) and might be able to influence the matches a little as well, secretly using her mom's name. The Shah, of course, would not know she was doing this. The girl wouldn't tell the slaves who she was, either, in fear of some of the more dramatic ones turning around and murdering her. In fact, it's possible that many of them wouldn't even know that there was someone like her working for them. My assumption is that the leader she trusted the most would know her role in the whole thing, as well as possibly her heritage. In fact, he/she would probably have been the one to tell her not to tell the others.
She doesn't tell her mom for two reasons: first off, her mom wouldn't let her out or do anything if she knew what the girl was up to, therefor rendering her helpless for the rebellion; secondly, while she might not appreciate her mother, she still does love her as a mother. So there would also be some interesting conflict there, especially if we had someone to play the Shah.
Now for how she reaches to the slaves. This is all assuming they live in the colosseum, which I'm still not positive about (I didn't see anything about it on the information page). Of course, if they don't, I can adapt it to fit. She tricks the guards (or whoever she would need to) into believing she's allowed to see the slaves. She explains to them that her mother, despite her attitude, actually does care about the slaves, just doesn't want the common people knowing about it in fear of them striking her down from power. So the girl says that it is her job to check the slaves (and, of course, that her mother doesn't trust anyone outside of her bloodline to do such a thing) and make sure they are being treated as slaves should be treated. To do so, she explains, she must also go in pretending she is a slave and none of the slaves should know it's a lie. That was they will answer her questions as normal and she will know the true situation of their lives. She also makes sure that it's clear that the guards mustn't tell anyone about this, just in case someone overhears. That includes her mother, seeing as she is always surrounded by guards and the like, all of whom are unaware of her true concern. In other words, she just comes up with a lie to make the guards let her in and understand why she has to come regularly.
As I said before, feel free to shoot this idea down completely! It just came to my head so I figured I'd propose it.