Is there a way to ferment mustard seeds for homemade mustard?
Whey appears to be an agent for lacto-fermenting mustard.  This recipe for lacto-fermented mustard describes a process of combining ingredients and then letting them ferment after the recipe is put together by leaving out over a few days.  

This recipe for a mustard seed raita uses yogurt as the fermentation ingredient, which means that you would have to use yogurt with active cultures.