Would it be possible to install a mechanical wastegate with a blowoff valve on our cars? I think you can do this by welding the flapper on the turbine side shut, and installing a blowoff valve on the exhaust side...i think?
I thought a blow-off valve was a relief valve for the compressor side of the turbo. A BOV on the turbine side could regulate pre-turbine exhaust pressure, but boost pressure control would become variable. A wastegate is a relief valve for the turbine side that is signaled from the compressor side.

You could add an external wastegate, like a deltagate, but it would be a lot of work to accomplish the same thing as the internal wastegate. What problem is the stock WG giving you that you want to change?
Well, our dual port wastegate has a rubber seal in it, and i tore it trying to adjust the rod. I plugged one of the ports and it seemed to fix the problem, do you know why they even have two vac ports on them if you really only need one? I was just going to try somthing different with the wastegate. I just replaced the turbo about 1200 miles ago and the oil line that goes on the back had one of the bolts wiggle out causing the oil to go onto the back of the engine instead of back into the engine. Long story short....Fried Turbo. So I pulled it back off, and going to have it rebuilt again.