I would stop and shut the bike off, and give the riders the chance to move thier horse over to the right as much as they want.
If they don't seem to want to do that, then just give plenty of room and ease on by.
I think its a little ridiculous for them to expect dirtbike riders to take their helmets off. Unless I'm stopping to have a conversation, I don't take mine off.
I'm about 60/40 w/ equestrian people being nice or jerks. I'm sure some will kinda hate dirtbike guys just on principle. But I've also met plenty who were equally polite and civil.