How about a small turbo sat on its own, with its own fuel injector in a combustion pipe between the compressor and turbine? - Constant boost, fed into the other turbo or just into the intake via the I/cooler etc, no need for a turbo in the exhaust blocking things up then - bit noisy and hot and probably highly dangerous, but how cool would a constant 30psi with NO lag and no exhaust restriction? Just have a CO2 starter bottle to spin it up before it self sustains