its a feature in developmnent …
(you can actually make use of it already via the passthrough directive in snapcraft.yaml (note though that it will fail store review until the feature is out of experimental state))
you should be able to use something like (untested):
passthrough: layout: /usr/lib/x86_64-linux-gnu/webkit2gtk-4.0: bind: $SNAP/usr/lib/x86_64-linux-gnu/webkit2gtk-4.0