Update: finally received all the pieces and thought I would report on what I ended up doing:

Using a 52/38 front combo, and I created a rear cluster of 27/21/19/17/15/13, I ended up with a nicely effective 9 speed (out of 12 possible), with nice even jumps of 11-13% except for the 1st to 2nd jump of 29% (like a granny) and the jump to highest gear at 15%.

The 600EX Arabesque RD had no problem with the 27 tooth rear, and actually could handle the 52/27 combo (not that I'd use it!). No issue with chain sag on the 38/13 combo either.

Thanks all for your thoughts and advice!

Mark