I'd say two things about this.
1. Opening the native keyboard and plumbing those events through to the WASM runtime sounds pretty easy. It's probably not cause modern software, but conceptually it should be trivial.. right??
2. In terms of 'the boss' wanting to do 'that one weird thing' that there isn't a library/plugin/whatever for in DearImgui land. If dev time for everything else gets faster, than the 10x cost of that small corner case can be absorbed by net win. Now, I'm pretty sus on the claim everything else gets better today, but we can certainly imagine a world where they do, and it's probably not far away