Back to the question at the top of this page...

Can the S900 do the 'one channel MIDI input, treat as if it were the built-in keyboard' trick?

That's a seriously handy feature. ALL arrangers should have it, even the low and mid-line ones, IMO...

Combine that with customizable codes for triggering variations and fills (because there is NO cross-manufacturer standard, yet) and you have a pretty easy to set up two arranger über-arranger

My G70, plus the S900's Mega guitars and SA voices = possibly the best arranger in the world! (OK, T3 if I had the budget!)