Thank you Marius for this patch.
I've tried it and it seems to be mostly working.
I've tried updating PyQt to include webengine: also seems to be working.

Finally, I've tried updating Qutebrowser to 1.6.1 with Webengine
support.  I went quite far, but unfortunately on startup it shows:

--8<---------------cut here---------------start------------->8---
> /gnu/store/m94dlnqcln96v02x0z2hxsr50gsmn0g7-qutebrowser-1.6.1/bin/qutebrowser 
15:50:10 INFO: Run :adblock-update to get adblock lists.
15:50:10 CRITICAL: Could not find QtWebEngineProcess
aborted
--8<---------------cut here---------------end--------------->8---

I tried adding pyqt5/libexec to the PATH (in which QtWebEngineProcess
resides), to no avail.

Find my patches attach (it includes Marius patch).

-- 
Pierre Neidhardt
https://ambrevar.xyz/