Actually the Octopus does this already: You can select which notes should be in your scale by pressing notes on your keyboard in realtime. However when in this mode, each key works in the same way as the switches on the Octopus. This means that you press a key and the note is added to the scale, and you press it again and the note is removed from the scale.
This is not the way your arranger keyboard or typical arpeggiator works, but it is not far off.
I must admit that I would l-o-v-e to have a "classic arp" mode where the notes of the scale is selected by only the notes you hold. This would quite fantastic, and I hope that with a little sweet-talking we can get Gabriel to make it:kiss: