I have asked the same question of scale cops in two states; AZ and LA. Both gave the same answer. PrePass takes precedence. The scale in LA had a sign on the wall saying the same thing. Several states have signs posted before the scales saying to obey PrePass in-cab signals. Thus, in those states your PrePass signal trumps other signs that order all trucks to enter the scales.
The PrePass Plus web site FAQ page addresses your question in reverse:
"5) Won't I get pulled over for not pulling into a weigh station?
PrePass is authorized by state law enforcement agencies. Law enforcement officers are aware of the PrePassPlus program and will not be concerned as long as you follow in-cab signals."
The question is reversed but the answer is the same. Follow in-cab signals.
Link to PrePass Plus FAQ web page:
http://www.prepass.com/faqplus.htm