magnetic coupler on a 12 wheel tender?

I do not think so.  Not easily.  The axles are to close together on the six wheel truck.  The six wheel trucks mount the coupler on a drawbar that comes of the truck frame near the truck mounting pin.  The magnetic couplers all hang under the axles so the uncoupling armature will be near the uncoupling track magnetic.  You might be able to work something out with one of the long shank passenger couplers. 

   A magnetically sensitive switch, a reed switch, could be used to fire the coil coupler off of the rollers power.

   The shoe can remain functional. A reed switch is, or at least used to be, a very small usually (used to be) glass tube. They can be superglued, painted, insulated with shrink tube, "buried" in epoxy, or goo, etc. etc.

  You may have seen alarm system widow/door switches in plastic cases. 2pc,one on the frame, one on the window/door. Inside one of those pieces is a reed switch.

Rick, both David and Adriatic are correct.  As far as I know the 6 wheel truck only came with coil couplers.  It's design does not allow easy installation of later magnetic couplers. Using a reed switch, as Adriatic suggested, will work at some distance, since the magnet coil in the track is pretty strong.  I would not try to re-engineer the coupler to fit a magnetic one it just too much work for very little return.  If you already have uncouplers in place I would just uncouple the car from the locomotive or, you can always use "the big hand in the sky" to uncouple the cars.

Maybe I'm missing the point.  I agree the original trucks were only electromagnetic.  However, you could replace those trucks with more recent Lionel 6 wheel trucks with mag couplers such as used on the many traditional engine's of the last 30 years.

  You know I'm not fussy. I'd use whatever was closest on the bench.

   But the point (if there was one ) was keeping it original as possible with a mod that would be cheap, simple electro-mechanical, easy to wire and reversible/ repaintable if done nicely.... mostly cheap though 

  I've never measured a coil couplers draw. You may need a relay too as reed sw. max out at about 3 amp. One used to drive a relay (or transistor) would allow a smaller reed sw. and they can get quite tiny.

Over sensitive ? Insulate/ field shielding/positon (rotation affects its radial reaction abilities, which are directional radially)

I think most reed switches are still less than a buck. So if the 3a works it is dirt cheap. Wort case, a couple more for a relay to pass the coils current... still cheaper than two trucks.  I'd order extra reed sw to have around. Like a bulb, you will likely break one or two sooner or later. ( coating a reed sw. in grease or oil before being buried in epoxy makes replacing buried ones surprisingly easy. Slice open with a dremel disk pry out with a pick, and repack with fresh epoxy. The old epoxy actually leaves a good seat this way) )

Thanks for the replies guys.  I have some old reed switches laying around.  I like this idea.  I could use the reed switch to pick a small relay that supplies power to the coil in the coupler from the roller pickups and even eliminate the sliding shoe if I wanted.  You guys are brilliant.  Thanks again.  I appreciate the help.

Take a look at a Lionel 9011-100. I expect it would drop right into your 2671W tender.