Good morning,
How big of a piece is it? I'm guessing you want to burnish something the size of a motorcycle seat? I've seen saddle makers use a tool (I believe it's usually called a bouncer) that is a round knob on the end of a stick. They use it like a bone folder to work the seat of a saddle down into the tree and into the shape they need. When I've seen it done the leather is very wet, just like wet forming, and it doesn't seem to burnish much. However, I think if the leather was very dry, like for tooling, I think it mightburnish it. The key will be applying a lot of pressure when you do it. When tooling I can burnish a small area with the rounded end of my stylus though it's not a really dark burnish like you get when tooling. I don't think it will be easy to do a large area that way. If you can find a small smooth round knob and rub that over the leather with a lot of pressure it might work.
Or, do you have a press? A smooth flat plate might work if the leather was cased but pretty dry.
for what its worth,
Bob