I don't have any equipment that would allow me to test it, but I know that some equipment will present multiple DLNA Renderers based on the output they have selected.
That is, in your case, if you selected Headphones on your device, MC would see one DLNA Renderer, and if you selected Speakers, MC would see another DLNA Renderer. That would allow you to associate a different DLNA Server to each DLNA Renderer, and hence, have different DSP for each.
Obviously, one Zone is one DLNA Renderer (Dynamic Zone) which can only have one DLNA Server associated with it.
If your device only advertises one DLNA Renderer regardless of output, then I don't think you can do what you want. Maybe the device has a setting to define how it presents?
The only other option I can think of would be to use hotkeys or similar to switch DLNA Server association with a DLNA Renderer. I haven't looked to see if there is a MCC or MCWS command for that. If not, maybe a request, but the switching would be very environment specific so may not be possible, or easy.