This time the majority of US citizens are not able to participating in the icos. The possible thing if those are participated in the ico used fake identity. VPN is useless right now because the majority of icos are giving obligation to the every ico participants to filled the KYC requirements to prove of they are not from US or another countries banned icos.
I don't see any possible way again. Another possible thing for US citizens to buy the token or coin through exchange site or service.