Your desired workflow should actually work. Once you have a running REPL, you can re-evaluate any namespace (using Clojure's
load-file or whatever is provided by your editor tooling) and it will actually re-load that namespace on the fly. Restart your Arachne runtime and you should be good to go.
For more extended hacking, you can temporarily add the
/src folder of the module you depend on (core or otherwise) to your application's class path. This even lets things like repl.reloaded do their thing.
That said, you may run into issues with protocols and records, if the namespaces don't load in the right order, or something is left with a dangling namespace. Thats an artifact of reloading things in Clojure, though, and isn't Arachne specific (other than that Arachne makes somewhat heavy use of protocols)