Keith sometime plays songs in different tunings, Midnight Rambler is one of them. Usually he plays it in standard tuning capo'd at the 7th fret (easier to play it "properly" this way), but there's footage of him playing it in open G, 5 strings & capo'd at the 4th fret.
I use my 5 string open G tuned tele to do Status Quo's "Rain" (other guitarist in standard tuning). I believe that open G tuning is actually a banjo tuning.
While on the subject of tunings, I do ZZ Top's "Tush" on a (mostly) standard tuned guitar, I drop the 5th string from an A to a G. Not sure if this is how the Rev Billy G does it or not, but it works well for me. Cheers