For cars that have air-bags in the seats, opening the seat up and placing a heating element and other items in there could have a negative effect on the safety of that seat - airbags might not deploy properly etc.
Even stiching the seats back up might also compromise safety, as the stiching might not allow the seat to 'open' properly when the bags deploy, or mal-function potentially causing more harm than good.
If you are wanting heated seats why not go out and buy a fluffy hot-water-bottle and whack it on the seat before you drive off - cheaper too, and you might be able to find one that matches the colour of your skirt.