This looks really cool. Applying a thin layer over parts of the standard library is a very good idea IMO. Reduces the dependency tree but a lot.
One question (my Swing knowledge is at least 15 years old!): what's the simplest way to handle events using this? Eg taking the FormTest [1] example, how might one simply bind a model object to the UI elements?
[1] https://github.com/HTTP-RPC/Sierra/blob/master/sierra-test/s...