As the guy who originally came up with the whole store-maps-in-PNG-metadata idea, I'd say you shouldn't feel bad about abandoning it.
It's probably easier to move to a zip-based format. All map layers are PNGs in the zip, referenced by a text file that contains all the entities, too. A new Garrison Builder would take care of all the messy details, of course. I had a lot of ideas spec-ed out way back when. I can try to find my old notes and give them to you if you'd like.
For easy map sharing, you could make a map-sharing webapp (maps.ganggarrison.com or ganggarrison.com/maps?). Users upload their maps and get a nice URL where people can view a written description, see thumbnails, and download the map. Garrison Builder could generate the thumbnails and store them into the zip, meaning the webapp wouldn't have to do any fancy rendering.
Have you guys considered setting up a Continuous Integration server? Every time someone commits to master on Github, it can automatically download the code, pack it up with py2exe, and upload the result onto a website. That way curious folks can always download the latest, bleeding-edge build and play around with it.
Just my 2 cents. Keep up the good work! Hope I'm not derailing the conversation.