No answer to your question, but I used to also have this problem. I'd used Flickr prior to Aperture, and would use Sets and Collections to create all sorts of groupings of my photos for easy browsing. When I started using Aperture, I did something similar, creating hundreds of projects, but I found organization the growing number of projects frustrating and had similar concerns about keeping images organized on Flickr and/or other non-Aperture setups.
I eventually decided to ditch my event-based Aperture Projects, and instead did a Project for each season of each year ('13 - Winter, '13 - Spring, '13 - Summer, '13 - Autumn, etc). I used Smart Albums to select images in the season's date range to help facilitate the moves from the old projects to the new projects, and when a project that didn't fit my new scheme had a theme I would add relevant tags to all of its images.
I then did extensive word, face, and location tagging within Aperture, and now find pictures using combinations of these, which works quite well for me. Most of my projects have a few hundred or a couple of thousand images, but I still find it quite easy to find what I'm looking for, and I can create Smart Albums that automatically contain images with specific metadata (Keywords: camping, Place includes: Colorado, EXIF Date is in the range 2006 to 2007). One of the really nice things about this method is that the filters are so flexible that once you have strong metadata you can easily modify existing filters to get precise images (e.g. "show me pictures containing my wife, my daughter and our dog that were taken at our old house").
I currently have a manageable 35 Projects, each photo has a Project (and only one Project) it belongs with, and I syncronize each Project to a matching Flickr Set. It's great that Flickr uses the same tag (keyword) and location data as Aperture, so you can generally do ad-hoc image filtering using this meta data to get the exact info you want. If I have a specific grouping of pictures I do have to double-upload them to Flickr, but I've generally found it's easier to use the synced metadata on Flickr to find particular groupings.
I know this may not work for what you're specifically trying to achieve, but as there were no replies and I'd had a similar problem but developed a new organization that works for me I figured I would share.
