How to use an alternate directory for config?

[jaraco]

In mercurial/evolve#503, I'm trying to add a tox.ini to the project, but that project would rather the file not be in the root of the directory and would prefer it to be instead in contrib/tox.ini.

When I move the tox.ini file, however, that means (a) it's not discovered without -c contrib/tox.ini and (b) assumptions about the locations of everything change (since {tox_root} is now contrib/).

I've found I can use changedir = ../tests in place of changedir = tests, and most things work like they did with tox.ini in the project root, except that the {work_dir} ends up being contrib/.tox, where I'd prefer it to be ./.tox. I tried setting work_dir = ../.tox, but that creates ../.tox for the environment creation, but that changes to tests/../.tox when running commands.

I tried setting package_root and work_dir, but had little success setting or referencing {package_root} (it never expanded).

Is there a straightforward way to reference a tox.ini but cause {tox_root} to be the current working directory even if changedir is used?