After a little checking on my own PS3, if you go over to System, then down to Audio Settings (or something like that) you should be able to select some kind of multi-output option, and also be able to select optical audio as the output method of choice. Between those two, I would think you could make it work.

Otherwise, a cheap alternative to a new receiver is a source selector. Adds a little complexity to the whole setup, but it lets you connect multiple devices to a single set of inputs. You can get these things for probably around $50 for a decent one. Compared to a new receiver, it's definitely a cheaper alternative, though not free.