Just to close the circle, I did eventually get around to trying some of the suggestions I got (over on the evolt list - another community I've not given the time they deserve recently). I wrote it up on my blog:
http://sam-i-am.com/work/2008/02/restoring-svn-repositories-from-disk.html
Basically, if you're doing svn via apache/DAV, you just use the SVNParentPath directive to point at your directory that contains your repo(s) - the parent directory, not the repo itself. And set the permissions correctly - and you're off.
Sam
who is not accepting comments on his blog template