The question is better answered by informing yourself of how dynamic libraries are found by applications on the platform, in this case macOS.
As far as I know, it basically comes down to:
1. Applications look in ~/lib, /usr/local/lib, and /usr/lib by default.
2. As you've found with Purr Data, an app bundle will also look in Contents/libs.
3. Applications will also look in other places listed in environment variables.
4. You can fiddle with the search path used per library when it's linked using install_name_tool.
See the docs:
or
or lots of posts on SO like
For distributing a plugin like an external via deken, options 1-3 are probably out to some degree. Option 4 however is not easy either as the path set by install_name_tool has to be relatively specific, in my experience. Getting it to work with the dylib sitting "wherever" on someone else machine is not so easy, unfortunately.
Gem makes this work, so I would check how they do it for macOS.
The other, simpler alternative is to statically link fluidsynth into the external. You then have a larger pd_darwin but no dependencies or dylib search paths. If homebrew installs a statically-built libfluidsynth.a to /usr/local/lib, you can use that.