[rotrex]

I have helped a chap implementing something similar. He bought my supercharged engine.

The fail safety relais of the HFS unit is controiling a boost control valve via a Audi N54 electromagnetic control valve.
This is a Rotrex install on a Rover K in a Lotus Elise. It is running a TB before the supercharger, so there is no recirc valve. As the owner wanted a boost switch (full boost and a limited setting), we decided to mount a blow-off valve (normally used as a waste gate on turbocharger setups) to the boost pipe between SC and inlet plenum. By venting the control line to atmosphere, boost is limited to spring pressure just like in a turbo build.
I was runing this valve before and it works as intended.

In your case, you could use the HFS fail safety relais to control the bypass valve.
If the current bypass valve has no active control, i.e. Is purely controlled by plenum pressure, you could add a N54 valve (VW/Audi part) to vent its control line to atmosphere upon activation of fail safety. If the bypass valve is normally open, it would open in this case. For the normally closed valve, I guess this is not the case as they are rather uncommen, the reverse logic would apply.

The HFS provides a potential free relais output, so any normal or inverse logic output at any potential, normally open or normally closed can be implemented.