Are canards useful for ultra low flying aircraft?
This highly depends on the overall conceptual design of the aircraft and there is no ideal configuration for any flight mission profile, except if you have very specific design constraints that will favor one configuration over another. In general, the trend when designing a wing of high aerodynamic efficiency, one that you would need for feasible human powered flight, is to have a traditional configuration of wing and empennage. This is for the wing to operate in clean air and not in the wake of the canard plane. Usually you focus on optimizing the wing and thus you want it to operate in its maximum efficiency.

A design example is the MIT Daedalus Human Powered Vehicle (HPV)


For further technical details regarding human powered vehicles and the their feasibility study you can start with this and discover further on your own.