That would be problematic. The repository maintenance scripts shuffle older versions of packages to the old section of the repository. If you wanted to build an older version you'd have to give the package a different name. Just R won't do. Something like R27 would work.
It's a bit silly to put an older version there, especially when the new one is already in the repo.
I'm not sure if rkward will compile against 2.8, though. At least, not without this patch that I think was never properly released.
Accept my apologies, but I won't be looking at it now, as I'm in the middle of something big until the end of the month.
But I will definitely compile it after that, because I need rkward as well.