This might be considered as common sense, but I think that this could help some of you out. I used the search function but I didn't find anything similar to this.
So back at season 10 I made some research and found this: Point Conversion Sellback Policy - Forums - World of Warcraft
It didn't say anything about not being able to refund honor as the new season went live.
So, a few weeks before season 11 was going to be released I started to farm some honor on my alt. Instead of keeping the honor gear I decided to keep refunding and buying the items back as I kept farming more honor. A few days before the release of season 11 I was fully geared plus all the items that I bought was still refundable.
When season 11 went live I simply logged my alt and I was indeed able to refund + buy the season 11 honor gear.
I'm not 100% certain that this will work when MOP goes live, but it might. So the theory is:
1. Buy full honor gear and keep it refundable before MOP.
2. When MOP hits just start leveling and every 1h 50 minutes just use your hearthstone / get summoned back to org and refresh the refund duration.
3. Hit lvl 90 and buy full honor gear.