Thanks for the links, but it does seem as if the actual text highlighting just isn't exposed.
As far as the actual issue at hand, I ended up opting for a different approach by eliminating the need for a text input altogether and using innerHTML with some JavaScript. Not only does it get around the text highlighting, it actually looks much cleaner.
This granular of a tweak to an HTML form control is just another good argument for eliminating form controls altogether. Haha!