Cooking lean pockets in toaster oven?
First and foremost, this. 

Second, according to the company their test kitchen tests all products in the microwave and oven and they have instructions for what they find works best. If the oven doesn't cook to a certain standard they only list microwave instructions. Note that all their products are pre-cooked so you don't have to worry about not fully cooking from a food safety standpoint. 

I suspect that some hot pockets have a higher thermal mass and cooking in an oven would easily lead to overcooked shell and undercooked interior. I looked at their web site and some lean pockets have microwave instructions and some do not. I would suggest cooking in the toaster oven at a lower temperature for longer and they should turn out fine. But let us know how it goes. Obviously don't use the sleeve. 

You may also want to turn up the temperature at the end of the cook to brown better after the inside is properly heated.