I'm not sure about Saab, but the Ford electric boost guages (like in the Super Coupe) work off of a computer vacuum/boost estimate.

I would guess you'd need some kind of electric vacuum/boost sensor that went to that.. call Saab and see if they can provide you with wiring diagrams for the car for freeeee and see how they implemented it.. if you even want to goto that trouble.

... my opinion anyway.