ah ok. Well, as much as I enjoyed Hesse's book, you have to bear in mind it is a work of fiction. The Buddha in this work has similarities to the historical Buddha when it suits the author's intentions and just as often deviates where it does not. In fairness, I don't think Hesse represented it as being anything else. So, though it is an inspirational work, it really has to be disregarded when it comes to assessing what Buddhism is and is not.
Is the concept that time, as you put it, "largely a mental construct super-imposed on our experience" restricted to the Mahayana strand of Buddhism, or does it extend further into other schools?
Well, seeing as the only living strand of Buddhism that is not
Mahayana is Theravada Buddhism, itself a rather homogeneous entity (compared to the Mahayana anyway), it is a bit more simple to answer. Or ought to be, I don't actually recall what their abidhamma says on this (retro? Someone else?).
But as I recall it, they don't really take time to be illusory. They are not really into the whole 'illusory and empty' thing with quite the same fervour as the Mahayana is.
But there have been some curious takes on this in the history of Buddhism. Such as the Sarvastivada (literally the "all exists theory/school"), one of the most dominant (non-mahayana) schools in ancient India. Here's a summation of them from Vasubandhu: "He who affirms the existence of the dharmas of the three time periods [past, present and future] is held to be a Sarvastivadin." Basically, they tried to explain how something like karma can transit across the three times by asserting that the past really exists as a separate entity and flows into an existent present, which flows into a genuinely existent future.
Actually, perhaps the two most prominent thinkers in Indian Mahayana, Nagarjuna and Vasubandhu, both made their names by debunking the theories of the Sarvastivadins. For that reason, many believe that in order to really appreciate their teachings, one has to understand the teachings of the ones they were largely reacting against.