In parallel with your synthesis Sound (and to the left of the MIDIVoice), you could add a SoundToGlobalController. In the SoundToGlobalController, you could use !KeyTimbre in the Value field, and in the GeneratedEvent field, you could construct an EventValue that depends on the particular voice number, for example:
!Timbre suffix2: ?VoiceNumber
Then you could use !Timbre01, !Timbre02, etc. to control various other parameters.