He wants to use it to advertise services that are not part of the P2P

protocol itself, but run on a different port. Reserving services bits
for those is not acceptable.

Why not? Does the port matter much?

All the NODE_EXT_SERVICES bit does is advertise the P2P "getextsrv"
command to get information, such as the port to connect on, for the
auxilary service.

Yes, I understand what it does, but from a clients perspective what it means is if someone implements a useful service and exposes it this way you have to seek out, connect to and interrogate every possible server even if (say) only a handful actually provide it. The most there's >1 "ext service" the protocol becomes extremely slow, vs service bits where you can download addr packets and see which IPs are advertising which services.

I don't see much reason to take a potentially large performance hit when there's a service advertisement mechanism that already works. What's wrong with the existing mechanism exactly?