So rather than wait around for someone to pass a Fusebox application my way, I realized that there actually was still an old FB3 sample application (all of one fuseaction) available for download from the fusebox site. So I grabbed that code, combined it with the necessary bits from a traditional FB5.5 skeleton and have created an upgrade path from FB3 to FB5.5. You can download the code below.
It actually wasn't tragically difficult to implement this. I know a number of folks seem to have been under the impression that this would be a big challenge. It turns out that there's a relatively convenient and simple way to implement this. There are a couple of things that contributed to this being possible (or at least easier). One is that the skeletons for these versions don't share any files. FB3 didn't have an Application.cfc and FB5 doesn't need an index.cfm, that was handy. Secondly if FB5 fails to find an event, it throws a convenient "fusebox.undefinedFuseaction" error that can be trapped and used as an entry point to run the FB3 app. Because FB3 runs entirely within the unused index.cfm, it's easy at that point to fail over to FB3.
There are of course limitations. You can't <do> an FB3 fuseaction from within an FB5 event for example. The point here was not to create a total mesh of the two environments, but rather to create a discrete way to support legacy code while continuing with future development on the newer versions of Fusebox. Personally I think this achieves that goal rather nicely. :)
For anyone interested in the code, the download is in the menu below (between the comments link and del.icio.us). The included release notes will tell you everything you need to know to get it running. Hopefully this will find its way into the 5.5.2 release of the Fusebox core. :)