I'm imagining the following version of Soundplane (let's call it Soundplane B) that I would consider ideal:
1) Several octaves of keyboard etched into one half of the surface, so that touching a key would generate that key's pitch - boring right? BUT then once the initial pitch is established, you could move your finger around to bend it on the y axis (assuming that's like up and down the keyboard) and control another parameter on the x axis).
On the other half of the Soundplane one portion of it would be an area to control (on xy axis) the intensity and speed of portamento, so you could control that in real time with one hand while voicing the notes with your other hand. And leftover space would be used for other parameters.
This would lead to a very cool performance controller, and then you could turn all of that off and just use it for regular Soundplane duties if you needed to.
The details are not yet fixed, but some kind of configuration software that can do what you are describing is planned for the Soundplane A. You could configure it to have a bunch of virtual keys, and then mark those somehow on the surface. Also, I have a very simple reference pattern planned that should be easy to learn your way around.