+1 on this. I don't see any obvious reason why this would not be a part of the stock functionality. It seems entirely logical that a filter set with two or more known states (i.e. dropdown, multi-select, swatches, yes/no) should be able to generate selector options within Layered Navigation.
I need this too, I just spent ages setting up products with yes/no only to find out later that I can't use this attribute for configurable products. The only alternative is to do it by adding a select list with yes / no.