It seems like it's been done a million times before, so why not a million and one? Yes, Slider Enhancer is a new jailbreak tweak that brings custom text to iOS' iconic "Slide to Unlock" bar.
Tweaks like Springtomize 2 and others already have such functions built in, but if you're looking for a standalone app that's free, Slider Enhancer could be exactly what you need...