I ran into this a little while ago, but in reverse; I was applying hooks to project repositories but some of them were identical, and it seemed inefficient to apply them redundantly. As it was, the practice of creating projects in SVN wasn't good practice: each project was given a repository in the root (the root couldn't have hooks applied to it.)
In my case, the fix was to have a central repository with project trees falling under that, which meant I could apply my hooks to the, say, Client
repo and have it cascade so it was applied to all projects under that root. This become much more effective, as I could then section projects off and apply per-repo-type hooks without redundancy.
In your case, what I think you want is the other way around, so in essence each project needs to be a repository, not just part of the root repository.