Is it possible to write macOS/Cocoa applications in Google Go?
Is there a Go-Obj-C bridge? (it seems to me that Obj-C dynamism would be a great fit for Golang's interfaces)
Can I at least link the two together and make them talk to each other via plain-old C functions?
CGo is what enables you to call C code.
See the CGo doc and the informative, official blog post on it.
There does not seem to be cocoa bindings/libraries yet, but you may want to check out the GTK package for reference.