I agree with many of the things said under this topic, and particularly with Scott's observations.
Many of the techniques he mentions are general techniques that each of us can learn in a good music school or learning piano (scales, chord progressions, arppegios in all keys).
Other techniques are more specific to the arrangers, like practicing a chord progression with the right hand and playing a bass line with the left hand (in split mode).
I also agree that choosing "your own style" for a particular song may be the best route to follow, specialy if the resulting performance has more soul and you "feel good" doing it your own way: the audience recognizes it quite often. I have several songs performed with styles not that close to the originals and they are well received by the audience.
But going back to the playing techniques, I would be very interested in knowing other arranger players' styles of playing. Here are the different playing "styles" (modes) in my performances:
1) One instrument over the whole keyboard, no drums (pianist mode), no automatic accompaniement.
2) Manual Bass (should I call it UD's playing style?

). The keyboard is in split mode. Drums, left hand is playing the bass line, right hand is generally playing a chord progression with rhythm (comping?). It's hard to play a melody line with the right hand in this mode, since the overall sound would lack of rhythm. So the melody line comes from the voice.
3) Auto accompaniement: the classical arranger style. Chords and an automatic bass line are triggered by the left hand, and the right hand play the melody or any improvisation of it.
A feature like the new "bassist" mode in the XD9 would allow a 4th mode, in which both the auto-accompaniement and a manual bass are allowed. Here, the right hand triggers the chords and than can do the melodic line.
Even the simplest playing mode (like the first one) can produce a great performance, if you play with heart and expression and if you understand the audience emotions. I had a fresh proof of this, during a big wedding party this weekend. The right tune in the right moment with the whole yourself, and everybody will be singing along with you, with the tears in their eyes, no matter the playing mode or style you're using.
-- José.