I've read Montana is an exception to the "at-will" employer status, but what if the employer is part of a federal Indian reservation, and abides by federal labor laws? Can the employer still follow "at-will" practices?

Montana recognizes the at-will doctrine if the employee is within their probationary period or, if the employer does not specify a probationary period, the first six months of employment.

That being said, the at-will doctrine is not a Federal law; it is a concept that is recognized by the Federal and most state governments. It has multiple exceptions.

You can check with the state DOL on your question but I very much doubt that the state of Montana will agree.