How much say does a liege lord have in a marriage between banner men? I got to thinking about how during the Hornwood crisis the Northern lords come to ask Bran/Rodrik/Luwin/Robb for permission to marry Lady Hornwood instead of just attempting to woo Lady Hornwood herself. Later several of Robb’s banner men offer to marry a Frey since Robb did not. I suppose it depends on the liege lord and if he is able to negotiate, since Robb did not command Edmure to marry Roslin, only convinced him?

I think the liege lord has a good deal of say, but it kind of depends. For one thing, it’s probably very different in a case of an heiress/widow without male relatives vs. with male relatives vs. a male heir/widower. 

I’m guessing that rather than outright commanding, it’s probably more the case that the liege lord has veto power (since it could be very dangerous if a neighboring kingdom starts marrying into your vassals) and can use their influence to strongly suggest that it would be in the vassal’s best interest.