Hey, ebookfab, welcome to MobileRead!
I'm afraid there aren't any existent text search hacks for the Sony Readers, and if it were possible to do via a simple hack, I expect we'd have seen one by now.
I can offer a suggestion that may or may not be helpful to you.
You can do the search of your text in the Connect Software on your PC. That will get you to the right page, and you can then re-transfer the book to your 500, which will get you on the right page in it.
It's not a particularly eloquent work-around, but it may be good enough for what you need to do, you'll have to decide that for yourself, of course.