Hi Mark - not to distract you, as its good to see someone pushing on the Spec integration with a real use case - but an alternate is a web application. Presumably you are familiar with Seaside but there is another kid in town that I���ve been having a lot of fun with with - CodeParadise - which uses MVP with the UI hosted transparently as minimal client side image (via the magic of Pharo Candle to create a minimal Dom based headless image that runs on the SqueakJS VM) using WebComponents. I���m still in awe over how this works and brings numerous Smalltalk technologies together (although - seeing Esteban demo a native spec app earlier this month was very cool too).
CP is still pretty heavily in development (so the same doc scarcity as well - https://github.com/ErikOnBike/CodeParadise) - but I���ve been helping Erik get Shoelace (https://shoelace.style/components/card) working in it - and that has a decent rich set of UI components that you can hook up (there isn���t a tree view though - not sure if that���s some CSS trickery to accomplish this).