I'm quite happy not to see their work 
The scenario is a one-off workshop, during the workshop one of the tutors can always come and look at a student's project on their computer, after the workshop there is no marking or follow-up. Moreover I'd like the workspace to be clean, ready to use again.
Now I've discovered from Using R Studio cloud for teaching package development that I can put a whole project in a sub-directory, I think the answer is not to use a workspace at all, but a single project in my "Your Workspace" which is a directory with multiple subdirectories, each of which is its own .Rproj. I've got this working here: https://rstudio.cloud/project/38916
This has the properties I want in that the top-level project gives the general setup (e.g. the packages installed that I want to use) and the student can open each sub-project and work on that as usual (in particular using the package build tools)
The only problem then is it doesn't seem to be easy to create these sub-projects. Any idea for example how I could clone a package repo from git to add as a sub-project?