Using the Book of the Entourage rules, is it still possible for a player knight to marry below his class by making a Loyalty (Lord) roll as in KAP 5.1? Or do all marriages roll on the Wife and Dowry Table on page 19?

I would allow it. However, note that you are ALWAYS better off by rolling on the table. Even the lowliest result is better than what you get in KAP 5.1 by marrying below your class. So to me, the loyalty roll represents more of a 'well, if you are really really sure you want to marry this commoner, even though you would have better, more suitable prospects...' from the liege, convincing him that this loyal knight deserves to make up his own mind. So I would use it strictly for story purposes, such as a PK having fallen for a particular Lady's Maid or something like that.