What I'd like to propose is a way to share more configuration among
repositories, setperf would do what i want but it has a too limited scope.

Something similar was put forward already:

http://lists.osuosl.org/pipermail/darcs-users/2005-June/007786.html

I was trying to make a repo easier to get today, the issue being that essential
scripts are not being set executable. For now i have created a kind of hack that
does what --set-scripts-executable should do. However, that still has to be
executed separately and is not obvious.

I am aware of the security issues mentioned at:

http://lists.osuosl.org/pipermail/darcs-users/2005-November/008777.html

but i don't think noop commandline parameters to darcs should be a problem in
this case.

This sounds doable and even potentially useful.

Suppose we invented a new setpref value like "permissionsfile" or
"symlinksfile".  Would older versions of Darcs choke on them?  

It may be acceptable if they simply ignored them, but if we could add new
setpref values, it would mean being able to add better support for things like
permissions (you could imagine a file that tells you which files should be
executable), a subset of permitted global flags, some sort of mechanism for
keeping track of symlinks, etc.

It wouldn't be going nearly as far as adding a new patch type, just sort of
handy functionality through the back door.

I think we need somebody to see how darcs 2.3.0 reacts to arbitrary setpref
patches and also darcs 1.0.9 for good measure.  If they both accept them with a
shrug (which I think they would since the code is pretty dumb), then we're in! :-)