In addition to the receiver not outputting the digital signal in analog, it turns out to be problematic to get my computer to output both analog and digital versions of the signal at the same time. However, I exchanged some emails with tigeraudio2007 off-forum (apparently the incorrect info about the receiver originated from his contact at Onkyo), and he figured out a pretty good solution for a good price. Interestingly (and surprisingly), it sounds like almost no receivers support outputting digital inputs on their analog outputs, except for some very high-end ones.
So the solution is to split the SPDIF output from the computer with a Y-cable, feed one end to the digital input on the Onkyo, and the other end through this:
and then into the analog input of the receiver. Only $80 + a Y-cable.
Much thanks for tigeraudio2007.