Ok, I'm just brainstroming here, it's most likely a garbage idea, but anyways:
If you are worried about distribution and want to implement a cap anyway, you could create some kind of daily or weekly cap, with some kind of background checks or a lottery system. Let's say, you want to have a maximum of 400 BTC and your ICO is supposed to be held for 4 weeks, that's 28 days. So, you could either implement a 14,29 BTC cap per day, a 100 BTC cap per week, or some kind of increasing system, with a 5 BTC cap first and so on.
You could combine this with a maximum amount that can be purchased per user.
This, combined with either a background check (Mail adresses, IP adresses, BCT accounts, whatever) or a lottery system (you register, if you "win" you are allowed to buy an amount you chose to buy with your registration) would decrease (although by no means stop) the likelyhood of few individuals buying uncomfortably big chunks.
Of course, you will always have those who a re able to game the system, there is no denying that. All you can do is to make it as hard as possible for them.
PS.: You need a more specific thread title