Not personally a contact, more of an assumption that his mother would know how to get in contact with the League given her history with it. If Batman is the senior surviving member, he'd be the guy that got the messages. 

I don't think there'd have ever been a time that Maxus-El would have met or spoken with anyone from the JL, though he would be well informed from a technical and historical aspect (i.e. perused detailed files).