I'm pretty sure this is more feature rich than dnsmasq, but the golang ipkg on the latest OpenWRT release is 99MB. My overlay filesystem started at less than 24MB. This software not for users like us. Perhaps someone will implement equivalent functionality using C or Python libraries for those of us that run by choice in a resource-constrained or embedded environment.

That's going to be the golang compiler - you can cross compile static binaries from your PC that will run on ARM. The final binary size should be a few mb.

I’ve got all UniFi gear but I believe it works the same. Where that doesn’t work is when using containers.

Thinking "aloud" here, but you might be able to get something like this with the macvlan network driver with docker. On FreeBSD I'm able to use a bridge interface that has a VLAN interface as a member (e.g. bridge10 with vlan10), and then have my jails create a vnet interface within that bridge. To the rest of the network it looks like yet another host (including showing up in Unifi).

if you run pi-hole you can do the same. Really useful for home networks