Hi,
nowadays we know that for the current Highmaul mission you need to have 14 (some still say is 15, I've seen it with 14) heroic kills, no matter what bosses to have mythic loot from follower missions. This number, I guess, is from bosses_in_the_raid * 2.
So, my guess is that for BRF you will need to have 20 bosses killed in hc in order to have mythic loot from the follower missions.
My plan is to farm one of the first bosses and kill it 10 times (plus the 10 bosses killed with my guild group will make 20 hc kills).
Does it sound too crazy or do you think is reasonable to assume this could/should work?