No, currently @custom only supports calling out to some HTTP endpoints. It doesn’t yet support full custom logic in which you can write a graphql± query directly. That will be part of 20.11 as @michaelcompton pointed out in the below quote.
But, at present, I think you can cut a hop by directly specifying the /query endpoint of dgraph in the custom URL. That would mean dgraph → dgraph, no resolver in between. See this and the following discussion for more info about how to do that.