I do miss having a heated front screen as my Lag doesn't have one. So much easier on a frosty morning, flick a switch and the ice was gone, no messing with a scraper or de-icer.
I did have a heated screen replaced on my last escort as some pesky kids booted a football at it and cracked it. The autoglass fitter showed me the bill for it, £380 just for the heated screen with a slight green tint, this was about 4 years ago now. Not bad considering I only paid £50 excess
Sure one could be retro fitted, hardest part would be sourcing and fitting a screen without paying silly money for it. As noel said, you might be lucky and find all the wiring there for it.
You could always try a highly elaborate and risky maneuver and tell your insurance company that one is already fitted and needs replacing!
I don't recomend that one though!