Zener diode hooked into the MAP sensor will limit voltage to 4.7V so you will never reach boost limit.
Cost: about 27 cents from an electronics store.
Obviously not as good as a proper engine management setup, but way more affordable.
I was going to do this to my Soarer, but won't be pushing the CT12A turbos past 12.5 PSI for quite a while. Needs more work done to ensure reliability first (like a new GT3240 turbo )