Hi,
It's actually the best way to go about it, it can be considered advanced math, but nonetheless, it's very very powerful and effficient to describe such layer mask using binary, so Unity engineers did actually very well here, they albeit forgot to properly expose UI widgets to access that conviently...
PlayMaker doesn't have any binary operation, but I have added this to my todos as with a set of custom actions, it's perfectly doable actually. Right now I think the best way would be to build the two actions "add layer" and "remove layer" for the walkable mask.
Can you get back to me in few days if I haven't done them yet? thanks.
bye,
Jean