Does Skeat actually mention 'rumbelow', or are you just guessing at a connection? Do have a look at Bruce Olson's comments in earlier discussions if you haven't already; most particularly Lyr Req: Hal n Toe. His references are mainly earlier, and make it clear that Rumbelow was at that time considered a place rather than a beverage. That doesn't mean that it was understood in this song in that way, but it does mean that we can't say for sure one way or the other.