Not the answer you are looking for, but I can definitively say that there is no configuration option to change where Rails finds the tmp folder. The location is hard-coded in many different places in the Rails database.
, symlink , , , .
, , , .. Dropbox , tmp, Dropbox.
, Rails ~/work/rails_project/<all the rails dirs including tmp>, Dropbox, ~/dropbox/rails_project, , , tmp one, bash, :
$ for f in ~/work/rails_project/*; do ln -s $f; done
$ rm tmp
, , / .