How is this solved in other cases (that is, in a way that does not prevent the resulting patch from being accepted)?
the way to go would be to no longer rely on winmm.SendDriverMessage, but to directly store in ACM structures the address of the DriverProc. In the case of acmDriverAdd, you would just store the function address, in the other cases you would grab the DriverProc address from the DLL. If you change this, you would be able to no longer export the driverProc from msacm and simply call acmDriverAdd for the internal msacm PCM handling driver.
A+