Notepad++ is one of the only programs I know of that does this, but that's not a hex editor, and throws a LOT of false positives (even with escaped hex-character strings :/), and since HxD is my primary hex editor of choice, being able to do all of this inside of HxD would probably cause the space-time continuum to malfunction!

While not a necessity, the ability to search for a partial-pattern (e.g. '00 ?? 11 ?? 22 33') would be nice, but definitely not a deal-breaker.
Thanks for all of your hard work!