Just finished reading an interesting paper given at NIME 2015 on some mods to the Soundplane that added "Vibrotactile feedback". Here is the link to the paper: https://nime2015.lsu.edu/proceedings/118/0118-paper.pdf
I love this project. I hope I get to try it one of these days.
Interesting read, I think haptic feedback could be a really interesting extension for expressive music controllers.
The paper never mentions whether the haptic vibrations reduce the sensitivity of the Soundplane by the nature of them being placed between the surface and the sensor. I'd be curious to know how much of the haptic vibration is picked up by the Soundplane's sensors, given that the haptic frequency is bandpass-filtered to 40 Hz - 400 Hz, and the Soundplane has the ability to read vibrations up to 500 Hz.
The closing remarks do admit that more research is needed, so I'd be curious to read a followup paper.