I think your odds are much less than 50-50 because I don't think they will accept your bona fides. The round-trip ticket will suggest you planned to do this. And you don't seem to suggest you're seriously ill.
If they do let you through, it's in the Chinese bureaucratic nature for there to be a cost, perhaps a ban on future use of TWOV.
If you're ethnic Chinese, expect even less sympathy from officials.
I'm inclined to agree. The "illness" excuse is likely to open additional cans of worms. While I agree that flying to HK and back is a big waste of time if you don't actually visit HK, this excursion isn't especially painful, and is preferable to pissing off PRC authorities. Next time, consider getting a visa.