I found this:
Lemon pepper is great for grilled poultry, but its most popular application is as a seasoning for seafood. In some cases, lemon pepper blends will include garlic so look at the listed ingredients when buying to avoid surprises. Some experts recommend adding a little salt to the lemon pepper when using it as a sumac substitute. When using lemon pepper in place of sumac, use about one and a half times the amount that your recipe requires for sumac.
I also found a substitute from Rachel Ray, of all people, where she zests an entire lemon, adds a tablespoon of black pepper, and a teaspoon of salt. She doesn't say so explicitly, but it sounds like that would be substituted one-for-one.
I should point out that I've never used sumac, so I can't speak from personal experience.