Unlocking the new Samson character requires one to "pass on 2 treasure rooms". I've recently started the game only to encounter an extra large version of the basement (the level I was on said "Basement XL"), in which there are two treasure rooms. Would ignoring both these rooms and moving on to the next level allow me to unlock Samson?
You definitely can, that's how I unlocked Samson. Pay attention that the game is still buggy after the release of the DLC, so it may happen that you unlock him without receiving any notification after beating Mom (also happened to me).