Thank you very much for putting this much effort into this and finally pulling together various patches flying around for years. I very much appreciate this and hope it will finally be included, so I have no longer to maintain my own branches.
However this probably also requires some changes to LUCI in order to become usable. At the very least it would be nice if there was an overview of leases from all instances. For convenience having the ability to configure each instance (as well as the "default" one) would be nice, too. Being then able to assign single instances of dnsmasq to a particular interface would also be great. All in all, I'm afraid that this requires a lot of changes in all kinds of places and it probably needs someone with more experience with the codebase to have a go at it. - are you interested in this :-)?