A guy in our club (National Capital Trackers) has/had a set up on some of his Postwar/MPC stuff. It was a bracket that attached to the lead truck on the engine, came under the cowcatcher and back up with the coupler connected to it. (think "U" shape ). No drilling, special modifications, etc to the body of the locomotive and could easily be removed with no trouble. If interested, I can send him an email asking about it. May have been a home brew job.
As far as I know, I don't believe any aftermarket couplers are out there. Certainly do-able but difficult, depending on the loco.