All that seems to be required is registering a dispenser behavior for each seed (and food if desired) using ether Botania or Quark's BehaviourSeeds class when they are present.

It could be useful if you explain the title more, since you just said how to do it and the title is a little wacky.

In Botania and Quark, dispensers can be used to plant vanilla crops. (By default Quark's version of the code auto disables if Botania is present.) I am asking that compatibility is added to Harvestcraft so that when Quark or Botania is present Harvestcraft registers its seed item (and possibly food item) to the corresponding crop block.

In Botania vanilla seeds are registered in SeedBehaviours.java and is Quark it is done in DispensersPlaceSeeds.java

(Sorry that I did not respond earlier, I was moving.)

judging by this it would be along the lines of "BlockDispenser.DISPENSE_BEHAVIOR_REGISTRY.putObject(Item.getItemFromBlock([seeds]), new BehaviourSeeds([seeds]));"