Custom List Field within a Template Admin interface?

[MonkeyTrainer12]

I would like to provide a List Field in a Template Admin that uses a value list based upon data previously saved using those same preferences. In a Site side form rather than an Admin, Ive done this before, using a Predefined List Class which was dynamically built when the page was drawn. Can a Site-side Template override an Admin-side Form Field Layout? Because so far, I haven't been able to make that work