If your use case is in a browser, then you want to leverage the ReplPad.
You could simply put the GUI in its own dockable window tab--which could sometimes be full screen, and sometimes share the screen with a Repl. See how the code editor widget was implemented. Instead of a code editor, imagine a graphics window there.
Unfortunately there's no way in ImGui itself to embed an iframe. So the Repl would have to be side by side or flipped to via a tab.
I believe what you want here isn't WASI. You want a WebAssembly "side module" written in C/C++ that the ReplPad can load into a tab (or allow to take over the whole screen)... and share the interpreter state between that side module and the ReplPad.
It does seem to have an active community. But my personal opinion is that Dear ImGui is very ugly, compared to e.g. TurboBadger.
However... I do think there are some applications for C++ GUI code that doesn't really overlap what browsers are designed to natively do reasonably well. So putting a Dear ImGui window in a tab and having it be able to make calls into the interpreter wouldn't be a bad thing to be able to do.
The trick though is that aspect of getting it to be able to make plain C calls into the libRebol API.