I thought I posted this the other day, but it doesn't seem to have shown up. I assume I haven't violated any forum rules in some way — at least, no one notified me that I have.
Anyway, I was working on some of the same issues not long ago, and since I have a license for HoudahSpot, I decided to adapt Jim Harrison's Project Notes AppleScript to create a HoudahSpot saved search file for the selected project. The script checks to see if a Finder folder hierarchy corresponding to the OmniFocus folder hierarchy containing the project exists and creates one if it doesn't. If the hierarchy already existed, it looks for a saved search file in the proper location; if it's not found, the script creates one and adds its URL to the project's Notes field. The saved search finds files with an OpenMeta tag equal to the name of the project. Of course you'll still need a way to apply the tag to the project files, but you can do that in HoudahSpot too.
Anyway, I was working on some of the same issues not long ago, and since I have a license for HoudahSpot, I decided to adapt Jim Harrison's Project Notes AppleScript to create a HoudahSpot saved search file for the selected project. The script checks to see if a Finder folder hierarchy corresponding to the OmniFocus folder hierarchy containing the project exists and creates one if it doesn't. If the hierarchy already existed, it looks for a saved search file in the proper location; if it's not found, the script creates one and adds its URL to the project's Notes field. The saved search finds files with an OpenMeta tag equal to the name of the project. Of course you'll still need a way to apply the tag to the project files, but you can do that in HoudahSpot too.