FYI - I submitted a feature request, and a problem report and also sent an email to Technical Support. I will report back here when I hear anything. Must be very frustrating as a developer to hit a wall like this.
Features like MFX (and CAL) are under-supported and it would be awesome if Cake would put in place the necessary support resources to stay on top of each of their features - (the infamous Staff-View also falls into this category.)
(I have very selfish reasons for this - I would love to be able to use some of Markleford's TenCrazy MFX effects that used to work so well in 32 bit versions of SONAR.)
As a minimum, I think a bit-bridge-esque wrapper for 32 bit MFX is needed. Ideal would be both a wrapper and a proper SDK so that 3rd party developers can develop 64 bit MFX for SONAR.
I am hopeful that other SONAR users can see the need/value of this effort and support it with their own requests to Cake.