I have absolutely zero idea how difficult this would be, or if its even possible to hook into the blizzard timer this way easily (or even if it'd just be easier to do it from scratch).
I'm looking for the same functionality; either count up to infinity or count down from a set point.
Key differences though would be the ability to resize it, specify font & font color, be borderless and have a transparent background (just plain text), and (ideally) pull from shared resources -- specifically fonts (though just being able to edit the source for that would be fine, too). As a fluff aside if it could do hundredths of a second that'd be boss as well.
Command line config would be fine, just an anchor toggle to move it around...
The goal is to just be able to create a very large (if desired) stopwatch/counter. Just plain text. I just prefer to time abilities by knowing fight duration, and as I have a gigantic amount of free real estate with which to fill with a hugely ostentatious timer, something like this would be ideal. Among other many mostly-pointless-but-peripherally-interesting-uses! The built-in one 'works', but it's oh so very small and not nearly loud enough. :(
If anybody has ever played Cabal, then the instance stopwatch timer in that is sort of what I'm getting at here. Although I know of no digital LED font for WoW for the full effect.
I am presently looking into what I'd have to do to make it myself, but as I've done zero scripting other than minor edits to existing stuff, it's very unlikely.