Is it possible to assign the Arpeggiator to the upper section? Hmm... I guess not...
If you can't change the assignement of the Arpeggiator at all, wouldn't it make much more sense to assign it always to the upper keyboard section?
I can only talk for myself, but in my case it definitely would! I usually use deep brass, string, pad, or lead sounds in combination with a arpeggio which should be playable with the upper keys.
There are two problems as result:
1. I have to pitch the sounds (up/down) to fit my needs.
2. It's kind of tricky playing the arp with my left hand while my right hand is playing the slower and deeper sound.
(It's maybe not saved in my brain to do it like that....