I am the author of mbox. Since I got too many emails regarding mbox, here are a few things to clarify.

  - naming: pkgfile mbox || echo looks like a good name
  - support: sorry, I don't have Mac or Windows.

I particularly like to use mbox for redirecting modification to another directory. For example,

  $ mbox -- git checkout file

You can checkout a file without overwritting the current file. You can imagine tons of usecases in this vein. Of course, blocking networks, restricting accesses of other directories, and rootless pkg installations are cute. However, to be a mature tool, I have to admit that there are lots of engineering works left -- support of 32bit .. compatibility layers .. still lots of corner cases.