This is a pretty big issue that I've not seen come up yet. In case you weren't aware, four of the ilvl660 epic PvP gears (head/shoulders/wrist/boots) that come out of Ashran are itemized for Mastery/Versatility. Since these two secondaries are vastly superior to any others for Monk tanks, that means success here is going to be pretty mandatory for entry level Mythic tanking.
Mastery/Versatility are so good that a 660 item is better than a 670 (Highmaul Heroic) item that doesn't have Mastery/Versatility. As such, when there are no Mastery/Versatility items available from Highmaul Heroic, the PvP stuff is BiS until you get Mythic drops. If you want to min/max as a tank going into Mythic, you need to PvP.
Mastery/Versatility legs/gloves/ring are available from Heroic Highmaul. Combine that with the PvP head/shoulders/wrist/boots, the legendary ring, and using a crafted weapon, chest, necklace, and cloak that you reroll to mastery/versatility, (cloak does not count towards 3-crafted limit) you can get a 660+ Mastery/Versatility item in EVERY slot. (except trinkets, but you obviously can't get mastery/versatility on trinkets)
However, to do this, you must succeed in PvP. The epic PvP gear requires 500 artifact fragments per item, which can be farmed from Ashran mobs or other players. However, Ashran has a mechanic where you lose half of ALL the fragments you own whenever you die, which means to get an item you have to survive for quite a long time, at least 100 HKs or be completely unmolested farming for hours. Ashran is a 100v100 zone that you can't steamroll with a premade. Considering how bad Alliance is at PvP, your only realistic chance to get this loot is to be Horde.