This doesn't make sense for the employee or employer. Per diem for travel is NOT compensation, it's reimbursement of a legitimate business expense that's designed to allow the employee some flexibility while giving the employer some cost certainty while reducing administrative overhead.

Travel is often highly variable over the short and long terms which makes tying your compensation to it an unnecessary and unproductive risk for both parties (my travel month over month can vary 100% and year over year by 50% or more). How would I negotiate that into my compensation in a way that makes sense for both parties?

Do you also have your employees they have to buy their own office supplies? "Really, trust me, it's built in to your compensation." (And why stop there? Why not have employees pay for the office utilities and build that in to their compensation? They are, after all, the ones using up all the heat and A/C.) Not very many high end people are going to want to do that anymore than they're going to want to be told that they're on their own for meals while traveling for the company.