No, the hit and target (it is called like in Nxt) requires only hashing. But as implemented the harvesting account is always generating a block (even when the node is not publishing the block) and that of course requires signing.
What task is solved by generating a block even if it's not published? If there are 2 accounts on a node, one of them contributes less into security of the blockchain and it's a waste of CPU cycles to use it for signing unless you have something special.