That is exactly what I have found...and it works 90% of the time assuming the guest singer has any sense of rhythm and doesn't get tangled up in the lyrics.

At a luau last weekend I had a guy come out in a grass skirt wanting to sing Little Grass Shack. He told me he did it in the key of E. I started playing. He grovled trying to find the melody...then the Hawaiian lyrics tripped him up. What saved the performance was, his grass skirt unexpectedly fell off half way through the song. You think that stopped him? Nooooo! He continued singing trying to pull up his grass skirt...and the audience thought it was the best part of the night.
Eddie