This question has been bugging me for some time, and I think I've found the answer. I can share two setups that I you can try on CT-S1 as a "backing pad":
MAIN: Piano of your choice (I've tried with default one so far)
LAYERED: VZ SEEGOD (doesn't sound by itself like something that can become pad, but it happens to work very well in the context)
1. Turn on the SUSTAIN function
2. Lower the LAYER VOLUME to your liking (try around 40-50)
3. Select the biggest Reverb of all available (Stadium, Cathedral or Pan delay are good starting points)
4. If you're using PEDAL it's a good idea to disable the PEDAL FUNCTION EFFECT on the layered part
Additional ideas:
5. Play with the OCTAVE SHIFT of the layered part.
6. Set EQUALIZER to Mellow
7. Set MAIN PART (piano) SUSTAIN TIME to lower value or to 0
Another much more subtle idea was to blend the piano with the sustained digital piano
MAIN: Analog piano of your choice
LAYERED: Any digital piano with naturally long tail, preferably without the strong FM-like transient
I've found these to work really well: PHASER E.PIANO, DYNO E.PIANO
If you can live with the strong attack part then: LUCENT E.PIANO or GALAXIA E.PIANO
Then you basically follow the same steps as above. The only difference is, to hear anything at all you might need to increase LAYER PART (e-piano) SUSTAIN TIME to the highest value (9).