I just tried the same setup as msabin: Bridging to a Wireless Network Card where all properties are disabled (except Virtualbox BRIDGING). The guest never gets an IP address. If I check the 'TCP/IP v4' setting of the Wireless Card, it does Work. I tried the newest test build as well as the official 5.2 release, I even went back 10 VB 5.1.30. Only the offical 5.2.0 release refuses to Work when 'TCP/IP v4' is checked.
I actually like the idea of isolating the guest like that, which is why I tried it. I followed Scottgu's post here:
https://forums.virtualbox.org/viewtopic.php?f=6&t=84176&p=399344&hilit=network+isolation#p399344
He also mentions that you can't do it with Wireless Network Card.
Are you sure this setup (with empty properties) has ever worked for you?