The traction control button is like the defrost buttons except it looks more the part. There isn't anything in those buttons to modify, they just send an electric pulse to the computer that controls whatever it is. There is a "chip" in the defrost buttons, but I don't know if it has the control for the "latching".
A momentary latching relay would work, but like you said it won't do hi/lo/off.
'03 SVT EAP "Eurotrish Bennett"