I’m not aware of any SFOS specific artwork (beyond the little in the docs repo).
You are asking for a solution that the entire software industry has failed to solve in a satisfying way (painful memories of mocking up UML in Visio circa 1998 come to mind …
But seriously, what tools are you using to make mock-ups? Does it encompass stuff like state / control flow diagrams or is it more ‘abstract’, just ‘organizing ideas’? If it’s the latter, I’d do pen and paper, first. A number of people I know use https://penpot.app/ … but that all ends up looking like crappy android/ios stuff, but it’s a very flexible design tool. What is the learning level of your students?
Frankly, I WOULD make mock-ups using QML and snapshot them (maybe automatically on state change). At least then you COMBINE the abstract modeling and initial plausibility testing Think ‘agile’