"Scratch space" under buck-out for end-user use?

Generally speaking I hate to pollute the top level directory with build artifacts, but every so often this is unavoidable for various reasons. On the other hand, I don't want to put things in `buck-out` if it might confuse buck because things appeared there. For example a shell script I have might download something temporarily and it's not invoked by `buck run`.

As an example, I have a script that starts buildbarn in a virtual machine, and requires me to create a "virtual" hard disk for the virtual machine to use. I kind of don't know where to put it, but `buck-out/` is tantalizing since it's already in my watchman ignore, sapling ignore, etc. (Making the shell script to start this a `buck run` target is probably possible even with the "loopy" nature of then needing to restart the daemon...)

Would it be possible or is it possible to have a a "scratch space" or "cellar" underneath `buck-out/v2` or something where you can just put your own temporary files? I.e. `buck-out/v2/cellar` or `buck-out/v2/scratch`, and the only guarantee is basically that 1) you can create this directory if you want and 2) buck will not touch it and so 3) you're responsible for cleaning it up.

(If this comes across as too odd or niche of a request I'm happy to take a "No", of course.)