The vet's first impression was the same as mine: it's probably an ingrown, infected quill. He decided to aspirate the lump. If it was filled with liquid, it should shrink quite easily. Unfortunately, the aspiration proved it was not filled with liquid but rather solid tissue.
The veterinarian viewed a sample of the lump under a microscope and found that the tissue appears to be made up of "round cells", which indicate a tumor. I decided to send the sample to the university for testing. A tumor like this can either be a benign mass that is easily removed or a sign of a serious cancer like lymphoma (unfortunately hedgehogs are prone to some cancers.)
I should find out the university's results later today. For now, I'm just keeping my fingers crossed that my sweet little hedgie girl will only have a minor removal surgery to endure.