[mwebb]

pumping losses or effects on engine operation would not matter
IF the system only functions during decel

max difference in pressure ambient to intake is about 13psi which is about the same as 26" of vacuum , during decel ,

max difference in pressure ambient to exhaust can be much higher and is not limited to the value of barometric pressure which is about
14.7 psi = 30"hg
at sea level
i have measured exhaust back pressure well over 20 psi during WOT snap on engines with restricted cat converters

using the engine as an air pump to pressurize an accumulator to spin up the alternator (or choice c something else )
ONLY during decel
will have little to no effect on the efficiency of the engine when running in a condition other than decel