Ed's got something of a point. Traditionally, lamb makes shepherd's pie and beef makes cottage pie. If I were competing my strategy would be partly based on that.
One thing for sure, I'd use hand cut meat, rather than commercially ground. This being the 21st Century and all, assuming you're competing with actual professionals, I'd make a composed, slightly deconstructed pie by making corn cakes and potato cakes, and "sandwiching" the meat between them. One cake flat on the plate, covered by meat (and sauce), the other cake tilted on top to form something like this: /_
Another way to use the cake idea is to make a single corn/potato cake as a sort of variation on Amish hash browns. I made that the other day as a garnish for grilled steak and wouldn't recommend if it wasn't good. I used corn meal to bind the cakes rather than whole kernels of corn, but the kernels would probably work as well or better.
For the meat I'd use a mix of lamb and beef -- with both of the meats slighlty pre-cooked -- either over charcoal to get some char, or over smoke to get some (you'll never guess) smoke. Then I'd prepare roughly as for competition chili. That is, cutting the meat into small pieces and preparing a stew. Unlike chili I wouldn't base the sauce primarily on tomato, but more on stock and beer. I would use some tomato paste as structure though -- probably starting with a pincage or perhaps even a tomato roux.
But if I did any thickening, it would probably be one of two ways: At the end with arrowroot -- it's a one-off which won't need to be held long after thickening, so you don't have to worry about it breaking; and you get the best gloss with arrowroot. Or, I'd thicken at the very beggining by adding some torn up corn tortillas not long after the liquids came to temp. It's not only another way of working corn into the dish, but probably pretty sexy for Three Rivers or wherever the heck you are.
The reasoning behind the partly cooked meat is not only to introduce some char and smoke but to nod at the origins of a dish which was created using meat leftover from the Sunday joint.
The idea of using sausage as part of the meat melange is very sound. You can bring some mature, ripened flavor mixes with sausage as well as smoke -- if that's something you can't do to the meat yourself during or before the comp. Chipotle chilies, and pimenton ehumado (smoked paprika) are two other ways of getting smoke into a dish without resorting to the polar extremes of actually barbecuing or using liquid smoke. Home made smoked salt is another killer ingredient.