I do loads of acoustic gigs where the feel still has to be 'up' & rhythmic. I change my lines when there's no drummer - I get a bit busier and play a bit more staccato. Flats, rolled back tone and sometimes palm muting gives me the note, and a nice percussive thump to drive things along.
I have my fretless, but IME the fretted works better 'cos the notes have more attack and bounce.
Also, when there's no drummer, the band should really work on dynamics to help with the light & shade.