In my younger days we made ruff looking beams using a small amount of blade exposure on a circular saw (wired up guard) then going diagonally over the made up beam...... No this is not what I want to do today....
I know someone out there has an answer so please share......
Way back when, I used to use a wire brush to take out some of the softer wood between the harder grain. It gave quite a rustic look. For larger pieces, some mechanical way to drive a wire wheel. Sorry I can't send a picture, the piece I made is long gone.
Best to you.