Hey Gorilla,
If you make a backing track of nothing but chords, then maybe put a BB melody (solo) in the middle, you're not duplicating the song. Many chord progressions are similar. So, should not be an infringement, since if you play it by itself, it doesn't sound like any song in particular. But if you want to sing in a lounge, music played there should be covered by the use fees, paid by the establishment, that are paid yearly to BMI and ASCAP. It's no different than if you were in a cover band working the venue. They almost never sound like the record. Kind of like a poor man's karoke, except it you want to validate yourself, you'll likely be playing your guitar and doing a vocal, or something similar. You're just having an electronic noise maker help you sing the song. The use fees should cover that, since for heaven's sake, you're not ripping off anybody. Cover bands do it, why not you. Except you're working your tail off creating some sounds to help you get through the song. This whole concept is a step down from being a DJ, but still allows you to sing and play. I didn't ask anybody about this, so if you're in doubt, ask other musicians who do something similar. There is at least 1 fantastic singer in my area who came all the way from Ireland to do exactly as I have described, and after several years, he's still playing the club he started at. He's not hurting anybody, not getting rich (well, I don't know that. Maybe the tips are good). Anyway, he's a pro, and well known here, and abroad. I know the club is paying use fees, as prescribed for any club. So, all should be well for you and me, as well. Hope this helps.
Griff