Thanks for filing this @stamblerre, happy to contribute a patch if I can get more information on where this change should live

Absolutely - that would be great! It would have to be in https://github.com/golang/tools/blob/master/internal/lsp/general.go - in the initialize and initialized functions, which correspond to the https://microsoft.github.io/language-server-protocol/specification#initialize and https://microsoft.github.io/language-server-protocol/specification#initialized requests. Basically, you'd have to check if the client supports dynamic configuration in initialize, and if so, do the dynamic configuration in initialized. Feel free to ask follow up questions if you need more clarification here.

It seems there is already a check in initialized to register some capabilities dynamically https://github.com/golang/tools/blob/master/internal/lsp/general.go#L130. With regards to the formatter, how would I go about registering it dynamically?

Yes, that's exactly the place to do it. It would look the same way, only you would be registering the textDocument/formatting capability.

Change https://golang.org/cl/185244 mentions this issue: internal/lsp: register textDocument/formatting provider dynamically

Closing as discussed on the above CL.