Versioning for testharness.js?

The Web Performance group is asking whether we can keep various
mercurial versions of http://w3c-test.org/resources/testharness.js.

ie, in order to prevent having the performance tests breaking every time
a change is done on testharness.js, they'd like to be able to write
something like

http://w3c-test.org/resources/testharness.js?version=<mercurial-changeset-id>

It seems a good idea even though I'd like to give some guarantees of
stability for testharness.js in a not-so-far future. We could even do
this on-demand, ie unless you request a specific version, we won't
bother creating the file on the server side.

How do folks feel about this?

Philippe