I mentioned thinking about this a couple of weeks ago. In between doing other stuff (and dogfooding when possible), the following "works for me" :

https://github.com/mdda/geany-project-tree

As the blurb says :

""" This plugin gives Geany sidebar a 'project-tree' view of your files.

The project-tree view can be different from how the files are laid out on disk : Personally, I prefer to keep a 'thematic' structure to a project - at the very least, it can be handy to keep files in an order not dictated by their sequence alphabetically.

"""

There's lots of stuff that it could do (and I may bolt on stuff, like find-in-files, as my own needs arise) - though I'm happy to keep the focus pretty narrow : There are many other excellent plugins out there, and many other approaches to projects...

Please let me know if you have any comments, fixes, or ideas! Martin :-)

PS: And many thanks to Oly - who was very helpful with code and other pointers