[eluser]dauber[/eluser]
Finally figured out the root of the problem.
First off, the actual issue was that I wasn't getting any return data from the jQuery's AJAX $.post(). Error logs showed that all the PHP code was working exactly as intended.
Then it hit me: it was because the button I was clicking on to get this stuff going was a submit button...which makes sense because my thinking was that I was submitting form data to do the search....except that when you use CodeIgniter's form helper and use a specific submit button, there's some kind of redirect going on. The give-away was that when I clicked that button I'd get a blank screen. I replaced the submit button with a standard button....problem solved!