Definition of mine-dragging:-
Here is my take on what Rain could have done to take mine-dragging to the next level:-
1. Firstly, by incorporating the use of a shuttle to drop a zealot on a field of mines (and in mid/late game with shuttle speed)
So, instead of sacrificing his whole or half of his zealots to the mines, which would be better used as a tradeoff against tanks, he could have incorporate or sneak in a few shuttles loaded with a zealot or two each at the position cluttered with most mines, dragging them and sacrificing only a few zealots before engaging an army of tanks and vultures head-on.
Here is an example of Soulkey (offracing and mine-dragging a zealot into an army of tanks, at 13:28 of the video below).
2. Secondly, by incorporating zealot with charge (or chargelots) onto the tank lines to draw the mines onto them.
Below is a short clip on Best mine-dragging using charged zealots against Upmagic.
Furthermore, Rain should have noted that mine-dragging with dragoons is only efficient when the said dragoons are not within range or proximity of the sieged-up tanks.
In conclusion, if Rain could improve on his mine-dragging, it could make his PvT even more formidable aside from being a feared opponent.