Update: After some more testing, I found that a Rannuncarpus planted directly above fully mature wheat will distinguish between mature and immature crops, and thus can be used to detect when the plant is fully-grown. Once I get some serious lag issues sorted out (probably by installing Botania in a small, easily maintained pack), I'll see if I can rig a setup similar to
this, using Bore-lensed spreaders or pistons to harvest the wheat crops the moment they grow. If that doesn't work, I should be able to use the Rannuncarpus to effectively count how many crops have fully grown. Once the blocks (probably cobblestone in this case) to be placed run out, the system would take this to mean that enough of the wheat has grown, and trigger some other mechanisms to harvest all the crops at once and reclaim the cobblestone.