Yeah, workshops are tough. In that case, http://rstudio.cloud may be a good (though in its infancy) solution! People can just create their own copy of your project, which may or may not use packrat for portability to local OS, etc. The spin-up-time is much faster for copies / forks than re-installing all the packages, though! Just not helpful if you are doing internal stuff for your company/institution.
As for dplyr, I think this discussion is useful if you end up needing other reasons to justify the rewrite to dplyr! Personally, I think the benefits are worth the additional dependencies.