Radium only has a Jack backend. If jack stops working on MacOS I will
certainly look into it, but I haven't noticed jack not working on any version
of MacOS (including high sierra), and I haven't heard any user compaining
about Jack not working on OSX before now.

Stefan, can you try to install Jack included with Radium instead of using brew?

I tried this version. It seems to be the same as on the jack…org/download page, right? It seems to work as far as your use case is concerned (started your radium and it seems to connect to that jack server). The ltc-tools (with jltcdump as I mentioned in another mail in this thread). Does not seem to be compatible with this version of jack though. I get the same “failed to connect errors” as before. 

$ jltcdump
unknown option character l
connect(2) call to jackdmp 1.9.11/jack-502/default/jack_0 failed (err=No such file or directory)

So, it seems like Jack2 works in some ways (as in your use case) but perhaps that the tools that link to its dynamic libraries are not always fully compatible. Or that the programming interface between jack1 and jack2 has changed over time, I don’t know.