Nice, looks well thought out.
Yeah the runtime thing is a disadvantage over type systems for sure. It might be possible with a NillablObject to do some kind of sanity checks at Rails initialization time. Also some other safety nets could help for example use of a ViewModel object to dictate some constraints on Views (type constraints and others).
I like Lucky’s table definition in the model class. A similar construct in AR might be useful in implementing some sanity checks. I often wish there was a way to get a table definition into AR while keeping the goodness of Migrations.
My colleagues wrote a library that does something like this. You may want to check it out: https://github.com/thoughtbot/wrapped