Yes I would have two signal pipes, one from before the turbo, one from after, then by putting them into the two different ports of the wastegate I would be effectively measuring the pressure differential between them and using that to open the wastegate, when it opens then it would allow air inwards to the inlet tract before the turbo (there may actually be a brief point where the inlet pressure is down to say 3psi of boost but the outlet is upto 18psi of boost where it might open very slightly too soon and allow charger boost to leak out for a few hundred engine rpm but I can live with that quite happily, once the turbo is spooled I basically dont care about the charger anymore until it unspools)