At Merton, at least, you can book people into Formal any night of the week, as long as they are going with a Mertonian (at no expense, either!). There are special guest nights which are followed by Coffee/Tea, and sometimes yummy choccys. As for the bar, Dave and Tony know all the Mertonians, so might ask who a non-Mertonian is with, but I've not seen that happen.