Comment 1 for bug 238864

This would take some work; additionally, it would only be useful for --source-only debs, because dkms looks in the /var/lib/dkms/<module>/<version>/<kernel> directory for the built modules to include.

You can avoid use of system directories by editing /etc/dkms_framework.conf to point at other locations for which you do have write permissions. I'd accept a patch to make it use an alternate configuration file.