Interesting question. I've never dealt with a problem like this, so you should probably take my advice with a grain of salt. The issue has come up in the forum before: http://moz.com/community/q/duplicate-content-from-multiple-sources-cross-domain. Matt Cutts also posted about this last year.
I imagine that your clients want all of their domains to rank, and to pull in traffic, so using NOINDEX is not an option. The simplest option would be to find a way to add some unique content to the pages - although again, at scale this might be a significant amount of work. More billable hours, that's all.
So, my plan of action would be to determine if this is really an issue in terms of ranking for the client (it might not be), and if it is, use the backend of their sites to display unique content on each site. It probably wouldn't hurt if the sites looked a little bit different from each other.
Hope that helps.