Unfortunately, this won't work because Bryntum components in the LWC run-time need to be rendered in an element marked as lwc:dom="manual" to allow direct DOM manipulation, which means that the Lightning Web Components framework is not going to be automatically re-rendering this DOM subtree (hence "manual"). So a custom element like this one won't be rehydrated.
Hi Aleksei, thanks for sharing your expertise!
I was thinking about empty containers marked as "manual" with delayed adding of components inside as it described in the article I mentioned. But didn't have a complete solution in my mind. Looks like we have not many chances for success.
It is unfortunate, indeed. For one of my LWC open-source projects I had to reimplement a couple of LWC components as Bryntum Widgets so I could place them in a Grid cell. Learned about the compose() method, it was fun