Option 1: Toslink to RCA adapter, then RCA to 3.5mm input on motherboard (not digital, but "lossless" is probably not what you're after, given most motherboard audio is pretty good these days).
Option 2: There are a few PCIe sound cards out there that have a Toslink in port (look for ones with two Toslink ports; one in, one out).
Option 3: The HiFimeDIY UX1 is both a USB to Toslink "DAC" (technically it's digital to digital, I know), but can also act as a Toslink input (over USB).
Option 1 is, essentially, what I'm trying to avoid. In my specific case, it wouldn't be much better than to just use the RCA cables from my PS2 itself.
Option 2 might be what I'm looking for, but I'm gonna need to go for a different motherboard for my desktop. Not a big deal, though, as I plan on going for an ATX size swap in the next couple of months. Option 3 might also be more practical than option 2 as it leaves a PCI-Express slot open for me if I went for 3 instead of 2. I'll look into the latter two options, then.