>>>>My recommendation: we are now sure the Hookworms are the tires you really want, but I don't see that it has been confirmed that they will fit<<<<
EUREKA!!! Amazing how the answer can sometimes be so obvious that you totally blow it off!
Before I made the purchase, I followed all the guidelines and measured my frame to arrive at the conclusion that I COULD fit a 2.5" tire. Then somebody reminded me that the side-to-side measurement wasn't the only one to be concerned with. With no top-to-rim measurement to go by, I decided, at that point, that buying online and hoping for the best was not the way to go with this. That's when I started calling around to all the local bike shops to see what they had in stock.
For some reason, once I installed the Schwalbe 2.35"s and saw that they fit, I completely stopped thinking about whether an even fatter tire would fit. Like a spoiled kid, all I could think about was how I really liked the "look" of those big, fat Hookworms and their awesome, matte-finished rim-to-rim texture - and how cheap and WalMart shiny the Big Apples looked. Even the fact that I had to remove the vertical support on my rear rack to accommodate the 2.35"s didn't raise a flag.
Anyway, what I'm saying is that, Rube Rad, I owe you a drink the next time you are out here on Long Island! Right after I read your reply, I ran downstairs, grabbed a tape measure and put it up to the bike with the Schwalbes installed. Before I could even take a measurement, it was so freakin' obvious to me that a 2.5" tire -- even a 2.4" tire -- could not possibly clear my rear seatstay. In fact, it's questionable whether one would fit into my front fork, either. I don't know where my head has been, but I feel like an idiot now.
However, I also feel A LOT better about the Schwalbe Big Apples, as they are now the best option I had for fat slicks. I wish the weather was better here on the East Coast, because now I'm not afraid to get out there and scuff them up while giving them a good test. I really look forward to springtime, though.
Thank you all for your advice and patience. This matter is now officially CLOSED!