Is Zakat obligatory on lent money that will not be returned until after a year (hawl), given that the time for Zakat payment falls due before the money is recovered?

The lent money is subject to zakat because it is your property. You have the choice between paying its zakat along with your other money when its (Zakat) year is due (which is more cautious), or delaying its zakat and paying the zakat for each installment you receive for the past years.