I didn't get a visible spore print from the portabella mushroom (it was store bought, so who knows how long ago it was picked), but I went ahead made a spore syringe from whatever residue it left and inoculated a liquid culture jar with it. A few days later, and something is growing in there. It's going to take some more time before I can tell if it's portabella mycelium, or some kind of mold/bacteria. Fingers crossed.
If it turns out to be a dud, I'll see if I can find a fresh cap at a farmer's market. In the meantime I need to sterilize some growing medium.