Its out there. I remember seeing someone had coded it at another forum and made it available. I just can't remember where. Keep asking around here and other forums.
Don't know about thinkScript, but you can get Medved Trader and hook it up with your TD Amertrade account (both data and trading). It has Candle Countdown on all intraday charts, not just time-based. http://www.medvedtrader.com/