SOLVED

There's a 'Save as preset" button in the SetUp window but it only works if the Octane render window is stopped. You also need to render out and save a preview if you want a custom preview. It's a bit clunky as a workflow but I think that's down to MODO, not the plugin.

Is there any info on best practice for saving and reusing materials in MODO?

There's a thread on the Octane for MODO sub forum but the links are dead.

Posting here as I'm currently using the demo.