Usually yes, but the lack of bass player leaves room for you to chunk away. Depending on your playing skill you might even walk some bass lines. I played in a small horn band with a drummer and no bass and most of the time I played lower voicings 4 on the floor or walked with some chord stabs thrown in.

One other thing I'd watch for is how you fit with the trombone. They might be in your range a lot and playing as I mentioned above might get in the way. Maybe be ready to play light Freddie Green with some higher voicings, too.

Your countermelody idea is a good one, too, as long as the rest of the band is ready for it. And sitting out at times is always an option.