Yes @dimitarvp, with OTP 23 and :code.all_available() (tried it with your Behaviours module idea) it seems to be working, both from mix task and as escript executable! Nice 
P.S. Just saw your Load all modules implementing a behaviour in escript - #10 by dimitarvp reply and related PR. That’s mostly what I tried locally, thanks! I’m probably gonna form it a bit differently, since I don’t need that behaviour module except in CLI, but still, the idea works. Great work man!
Welp, if push comes to shove you can always try to re-implement :code.all_available() in OTP 22 and below but I wouldn’t recommend it. Those are implementation details of the BEAM and any hacks we could come up with (adding ebin/**/*.beam globs and such) are not and should not be guaranteed to work in the future.
Happy to help! 
I sent you a pull request implementing what I meant. both list and sync commands work, (though I commented out a lot of code instead of configuring the repos, did not see the makefile before reading @dimitarvp posts)
Nice, thank you! That’s protocol demo by lud · Pull Request #2 · elvanja/escript_testbed · GitHub (for reference). And you were right, it is abusing the protocols indeed There’s even a bit simpler way with protocols, added to comments on the PR. Cool, today I learned!
That’s different because your trick is just to list all the modules that implement the protocol and dispatch a call. If you can do that, you can do it with behaviours too, no need to “resolve” them at runtime, the whole point of this topic.