figured it out again, sorry for the noise. This thread on using localhost helped, I used Brave first, which by defauly disallowed localhost.
I’m all set now for this stuff, but might be worth updating the docs for slow pokes like me 
2 things that would have saved me/will save someone a lot of time:
- explicitly saying the project within Plasmic will reload, and any registered components will appear at the bottom of the ‘Components’ section. Or maybe an in-Plasmic notification that the app host is recognized?
- a note that some browsers, like Brave, will block accessing localhost.