So I propose a Prospect command: You designate a single square of ore, gems, or economic stone. The miner then mines that square. He then checks all the squares adjacent to that mined square. If any of those adjacent squares contain the target ore/gem/stone. Then he designates those---on his own---for prospecting.
That simple algorithm should carry the miner to the end of the seam. And if you enter the seam in the middle then you prospect both sides, and eventually the miners will follow it to both ends.
This would reduce a lot of tedium for me, and I assume for many other players. It wouldn't replace the current mine command, because there are lots of situations where you wouldn't want the miner to follow the ore. But in the current version my miners are spending less than 20% of their time carving grand halls for living, working, and defense. All the rest of my digging is exploration, and then tediously following out each vein square by square.
only takes 1 or 2 screens per 'layer' (i.e siltstone layer, gabbro layer, and so on) even on the smallest fort size for you to have enough of all the ore your fort has available to last forever.