I've been wondering why a coldside (think intercooler piping) wastegate couldn't be used as a secondary failsafe for a system with boost creep issues. It would have to limit to something above the spring value in the standard wastegate otherwise the standard wastegate would never see full boost and wouldn't completely open.

AKA something like 8psi on the standard spring and 10psi on the coldside.

 

While not the most efficient method (or the most elegant for that matter), a cold side wastegate (check valve) would function to limit manifold pressure. This method does not solve the fact that boost creep is occuring, and a lot of energy will be wasted as you expel charged gas through the cold-side wastegate. The proper method to solve boost creep is to avoid it in the first place via an adequately designed manifold and properly sized wastegate. Or use a smaller turbo, more restrictive turbine or turbine housing.