Thanks for the great analysis @jamesh - a D-Bus API sounds like a good approach and would allow easy access via snapd confinement. In terms of the host-side, my preference would be to try and do this via portals so that we get the desktop integration parts “for free”.
I was also thinking whether this could be done via a content slot on each browser snap - but then this would really only work for snaps that want to use native messaging - it wouldn’t work for non-snap applications on the host.