Paul,
I went with the 1.5.x library layout for two reasons:
1) This was originally meant to be just between the MCP2515 and the Due, which only uses the 1.5.x series
2) The 1.5.x library format is the way Arduino libraries are going for the foreseeable future
One of the things I discovered is that the new library format is very particular about how files are laid out and formatted. If the library.properties file doesn't parse correctly, the library is unavailable to the IDE. Also, if you cut and paste the library.properties file from another library, forgetting to change the name of the library effectively makes it disappear. The "legacy" library format is much simpler. For my testing with the Teensy, I simply took the contents of the src directory and copied them into the top level library directory, and then copied over the examples directory.
Would it be helpful if I created a 1.0.x branch in the repository to push changes into? Or possibly the other way around, with master becoming a 1.0.x-release branch and then having a 1.5.x-devel branch as well?
-Neil