Yup, they have a service around 8pm. Pretty empty - a few monks, a few pilgrims, and a few locals - but worth it. They stamped credenciales afterwards, if you ask.
There are a number of other monastery-albergues along the way. I stayed in Markina-Xemeingo, Cobreces, and Cornellana, and there's one in Zenarruza as well.