Well, you can always bypass limitations with VPN or any proxy of sorts*, but if the ICO requires identification to confirm that you aren't using a proxy to bypass a supposed IP block on the website, they will still see that you aren't an US citizen. So the next step would be to use a fake ID... which is ridiculous. Why would you want to get into so much trouble to get in some ICO. Just stay legal. Find any other ICO's that aren't hosted in the USA. I think most will stop happening in the USA because ultimately what the ICO guys want is the entire planet being able to participate in order to get as much money as possible. But with less restrictions comes easier ways for scammers to scam, so do due dilligence and you should be ok.
*Even if you used a VPN, once you want to cash out, you would need to explain where these gains came from, so they would see that you are an US citizen that bypassed a law with an VPN... this could lead to problems. If it's not an huge amount I guess you can get away with it tho.