To displaying graphics telling a driver got drive-thru penalty, GPxPatch should able to do that better. Well, GP4 Tweaker is just a simple tweaker, not a graphics editor. :-P
Maybe the simplest solution by changing black flag texture graphic (located on every track wad files, I guess) with something similar with this purpose. But if just a text displayed on screen (something said like "You Got Drive-Thru Penalty", I can do that.
For drive-thru applied for keyboard controller, due to way of GP4 switched controller handling, I'm afraid that is cannot realized in short time. If this feature forced to applied to drive-thru, you'll lost all keyboard helps while driving.
To corn_freak:
Can you explain your problem more detail?
Gr88ts,
Madman