Trying to post inline in GroupWise, apologies if it ends up looking
like crap... 

> I*m imagining something where each
> contributor/accepted-pull-request-submitter basically just puts a
> digital file in the repo, once, that says something like *All the
code
> I*ve contributed to this repo in past or future, I have the legal
> ability to release under license X, and I have done so.* And then I
> guess in the License file, instead of saying *copyright Original
> Author*, it would be like *copyright by various contributors, see
files
> in ./contributors to see who.*
 
I wouldn't suggest imagining new things when it comes to legal issues
;) 

I would suggest considering the Developer's Certificate of Originality
(DCO) process as adopted by the Linux project and others (including
Evergreen). When Evergreen was in the process of joining the Software
Freedom Conservancy, that process was considered acceptable practice
(IIRC, the Software Freedom Law Center did take a glance) - no doubt in
part because it is a well-established practice. And talk about
lightweight; using the git Signed-off-by tag indicates that you've read
the DCO and agree to its terms. 

For a recent discussion and description of the DCO (in the context of
the Project Harmony discussions which were focused primarily on the much
heavier-weight CLA processes), see
http://lists.harmonyagreements.org/pipermail/harmony-drafting/2011-August/000099.html
for example.