I am trying to use this suggestion to suppress AdBlock for certain devices. Doing so seems to kill the DHCP server.
Do I have to wait for OS 4.0?
Yes, DNSMASQ does support tags. See A Kresd Journey for the details. I had this working when Knot was badly behaved a few years back. Like you, I turned on adblock at the network level and found I needed to divert some traffic direct to an external name server. Just figuring out the IPv6 bits now.