Electronic boost controllers
 

Can anyone suggest an EBC that's relatively inexpensive, has a fail-open solenoid and ideally has some kind of input to cut the boost when signalled?

While this isn't a boost controller, if you're looking for a boost fail safe, consider this.

Guardian Angel V2 Overboost Protection and BPV Controller [GUARDIAN_ANGEL_V2] - $215.00USD : Stratified Automotive Controls

They're located in Richmond and active in the Mazdaspeed/Ecoboost community.
I have their gen 1 and it works great.

I'm more wanting to be able to drop the boost to wastegate pressure if my wideband indicates my AFR's are drifting too far. It looks like I could make that Guardian Angel do what I want but having the setting in the engine bay isn't ideal. Definitely something to keep in mind though.

PSB-1 PowerSafe Boost & Air/Fuel Ratio Gauge
I have no personal experience with this though.