You wouldn't be able to fit a solenoid using a regular boost tee as the t-piece, vent and bleed valve are combined into a single unit.

I rigged up a similar setup, using a solenoid and a ball/spring boost controller.
Boost source from the turbo goes to the solenoid, which can switch between two outlets. One side is straight through, the other goes through the adjustable boost controller, and they both join up to a T piece before the wastegate. With no power to the solenoid, the wastegate sees normal pressure and acts like stock. Switch power on, and it has to go through the boost controller first so you can raise it to whatever you want. Works surprisingly well, and was under $100.
Only issue is the slightly complicated plumbing means the 'low' boost is closer to 9psi, than the 7psi it should be (and was before). But its very consistent on both settings.

ok I have been thinking again...
 
Will/should this work? it does in my head...
 
use a relay so,  http://www.narva.com.au/products/browse/relays
 
from the switch the +ve to pin 86 on relay
pin 85 to earth
pin 30 to the PWM digital output from the ecu
pin 87 to go to the MAC valve +ve
then earth the MAC valve -ve
 
so that way there will only be power going to it when the switch is on, and then should mean that when the switch is off, it only uses wastegate pressure???
 
am I correct in saying all that?
 
and what relay would be best to get if it will work?

I'm not sure you'd even need a relay. The solenoid won't draw much current, should check how much the ECU can handle and you can probably run it direct.

the relay is more for the available to turn it on and off