So what if I have mesh-based eyelashes that I would like to apply the same blinking shape key to. What would you say is the best/easiest way to do this? Having both eyelid and eyelash in one object, or make both separate objects and apply separate shape keys? I ask because sometimes when I tell blender to mirror the shape key, it doesn't do it to the eyelash, or if it does it isn't deforming right.
You can leave your mesh-based eyelash as part of the eyelid mesh. Modifications as a shapekey will effect separate mesh islands just the same. Simply select the eyelash mesh island and include it with the shapekey modification.
If you've already rotated the eyelid down as closed, with that shapekey selected, tab into edit mode and rotated the eyelash mesh down appropriately.