Originally Posted by
Macjet
Sorry, but no. An ATP can only instruct when under the training program of a Part 121, 135, or 142 certificate holder.
An ATP rated pilot can give instruction in “air transportation service”.
Part 1.1 defines Air Transportation as “interstate, overseas, or foreign air transportation or the transportation of mail by aircraft”.
With that said, an ATP rated pilot can give instruction in Part 121 and Part 135 operations but not part 91 operations. The instruction can be in any aircraft on the certificate holders list; it doesn’t have to be just aircraft requiring a type rating.
However, you're going to ignore this fact and log it any ways. So, disregard and press on.
The OP stated that the ATP had his MEI. So yes, he could log dual received under part 91. As to how it would look in an interview to show up with several hundred hours of dual given in this situation, that's a different matter.