Thanks again!
In parallel I had just found the “Low Level Widget Explanation”, which paints an overview of what is going on, and talks about the Comm channel – I’ll try to figure out how to access that (in the Javascript side).
If I manage to make that work (in the Kernel and Javascript), that should be enough to build a generic widgets framework in any language 
On design being unfortunate:
My comment was not about it being tightly coupled with HTML, it just needs to support updates from different ends, or communication between the different ends. Which, now I learned, it does, through this Comm channel. Looking forward to try to get it to work!