chroots/jails in jails

I'm making a build system for a project which creates a chroot in 
which to do some of the building to avoid base-system contamination 
(yeah I know lots of people do that).
the trick is that my test system is itself, a jail.
So I can not mount /dev in the chroot.

I can not predict where a build will occur so I can not pre-mount the 
devfs from outside the jail. (users may fire off builds in different 
locations)

Does anyone have any solution to this problem?

We have hierarchical jails, but no way of allowing the parent jail to 
give the child jail a devfs.

Has anyone looked at what it would take to make devfs "jail friendly"?

I'm guessing that the jail would have to get some devfs-rule parameter 
and that mount_devfs or it's in-kernel parts would have to know what 
to do..

seems like there should be someone out there who has hit this.. (and 
solved it?)

Julian