Graham said:

I do not think that anyone should have to know what subversion is in order to fix a bug in my R code, or change some assumptions, or parameters, or colours.

Sorry, I don't think it gets any easier that it is with subversion. Some computer literacy is required. Of course anyone can email changes to the authors who then commit it to the repository.
But if anyone is interested in and capable to work with R should also be able to get a subversion client running.