Unfortunately, I don't have any suggestions. Maybe DockMod is overriding it, but I glanced in the DockMod plist file and didn't see anything obvious. Perhaps that method just doesn't work anymore, although it seems like it should.
Well, I've only been using Mac for about a year so I'm no expert. I did check the DockMod plist file, but same as you didn't see anything obvious. I requested the text labels be removed (or option-able) from DockMod to the developer, so I'll report what he says as soon as I receive a reply.
Gotta say though, this is the functionality for the Dock that we have all been waiting for with Mavericks! $8 bucks is pretty cheap compared to other apps and I actually did receive confirmation from the creator that all plans are that it would continue to be supported (and FREE) with future versions of OS X. Thanks for your hard work and contribution to the Mac community Bryan!