Seems like it would be pretty simple to remove the resistance heat element (from the pictures it looks like a flexible pad, probably fireproof with the wire elements inside), that is sandwiched between the leather covering and the padding underneath.
If you get the new seats it would be a fairly simple operation once you remove you're present seats to disassemble the old seat bottom, pull the heaters, disassemble the new seat bottom, position the heater, reassemble the new bottom, and install you're new "heated" seat!
It has been a while since I pulled apart a couple of seats at my local boneyard to obtain newer/better seat spring boxes for my 126, I ended up removing the seatbacks altogether because it was quicker and easier as I was only after the springs, but because the seat bottom cover material is only held in place with a friction fit channel around the seat spring box frame, you may be able to simply compress the seat springs to release the tension on the material and therebye gan the slack nessessary to pull the fastening strip from the channel. You might be able to then fold back the seat bottom material and install the heater in place, and then reposition and fasten the leather back in place. With the entire seat outside of the car this should be pretty doable because you will have easy access to the underside areas. You'll need to position the wiring to make and keep you're electrical connections throughout the range of seat motion and that should be easier outside of the car.
One thing that I did take notice of in my foray is that the leather seat materials on boneyard seats can look decievingly better than they actually are, once I started pulling and stretching the material. The seats I worked on where pretty good looking but the leather was pretty dry and ripped in a couple places around the sides of the seat botton cushion. If I was lucky enough to find a set of better seats and would be swapping them over I would seriously consider going through a "Leatherique" rejuvination/cleaning process on the newer seats before working on them. I believe that softening and conditioning the leather before potentially stressing it during some disassembly/assembly work would be a prudent precaution. Should be pretty easy to do better and completely outside of the vehicle rather than after installation and may very well avoid the possibility of damaging nice seats before you can really enjoy them.
If you made sure all the springs where functioning and maybe even adding suplemental springs (there is a thread with the part numbers) you will be in great shape for the next 100 or 200K, or at least until your travels lead you to the "holy grail of seatdom" the factory option orthopedic seats that are out there somewhere waiting to be the next project!
Good Luck!
P.S. you might find some info or guidance from these guys
http://www.bergwerks.com/
http://www.bergwerks.com/seat_heat.jsp
Regarding the sewing (I'm a darn slow typist so I'm behind in the conversation) you might be able to alternatively position the heater element under the seat cover and then use something like high temp RTV to secure it in place if movement is a possible problem rather than stitching it in place. Also do you know if the heater is a single element on the seat bottom or dual elements on the bottom and the seat back?