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Idea is to solve long-standing X problem.
I know X support is in hard feature freeze, but lately someone added something like redshift support into KWIN for X11.

The idea is assembled from two part:
1. Watch for events about grab owner change and obtain information about grab (process/window/whatever)
2. Provide an KWIN api to register an grab

First part is to provide backward compatibility.
Second will extend functionality, so for example virtual machine manager could register VM window as grab owner and (in future) virtual machine could register window in VM as grab owner (needs create virtual hardware or some possibility to register it by writing into VM memory - I don't know much about Virtualization).

How it worked? After pressing one key, for example super, user could:
1. Select press super option
2. Disable grab
3. Select application/window, which will owning grab

About first part of this mechanism... We need to watch for X events, like grab obtained and get information about grab owner, where this message arrived. I don't know much about X programming, so maybe it's not possible, sorry... After message arrived, we could check we stored information about window/app obtained grab and include it if we don't store it previously, so user could select this application in future.

We need also watch for window/app destroy or grab revoke, but I think it's possible, because we are inside window manager process. There's only one problem. Where user select to change grab, X will probably sends event that this grab was unregistered. I don't know and that's probably worst thing to solve...

We must also provide an way to send fast grab change button to normal application. I propose fast pressing this button twice.