Duncan is spot on. I've submitted a very hackish patch, but it hasn't been accepted (nor replaced with something better), so as it stands, I'm not able to publish Hex packages (including "vanilla" Erlang ones) with the latest rebar3 and (unpatched) rebar3_hex. The plugin definitely needs an overhaul.
As for the release candidates, they currently don't work without some workarounds, due to the rebar3_hex bug (i.e. the Hex packages claim to have no dependencies and thus don't download them), but in testing locally with a patched version, I was able to use Duncan's updated lfex releases without issue. I've posted some comments and examples in the LFE Slack's #tooling channel, I believe.
I'll ping Tristan and see if we can't get rebar3_hex fixed up. After that, Duncan should be able to publish working releases for the lfex libraries, with minimal or no changes to the lfex code.
Re: rebar3 LFE support in general. There's an outstanding bug wrt LFE tests and `rebar3 eunit`. I'll ping Alisdair/Fred and see if we can't get that sorted too. This issue is simpler than the rebar3_hex one fwiw.