Using a "waste gate" would work and can work to limit and reduce the RPM. Yes normally the waste gate is used to blow off excess pressures going to the intake, but place a waste gate before the turbo and you can slow the turbo by now sending gas to it. See :
QTP » Electric Exhaust Cutouts
Instead of bumping the gas in to the air have a pipe run to a Y collector and rejoin the exhaust path just after the turbo.