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Unfortunately, As far as I know after looking into this some I don't know of this being possible. The scripts running the text editor aren't customizable like that and even if you were to change out the field with a some custom injected css it wouldn't work since the font-family is selectable. So that would force it to ignore the rule and become user-based anyhow.
The only way around this that I can think of currently would be to edit the submission and change it to the font that you want it to be. Another way would be to have it as a text area and just set the font for that field with a css based rule to Arial which should work also.