And now it's time for me to be sheepish:
It turns out I was was wrong about the SFX PSU. One would fit perfectly behind the PCIe and above / next to the 5.25" cage, the same place I was thinking of putting a Flex ATX unit. SFX is also two inches shallower, leaving more room for the expansion card, plus can have an 80 mm intake/exhaust fan.
This one seems like the best I could find in the 200W-350W range : http://www.silverstonetek.com/product.p ... 58&area=en
The same brand as The Egg pointed out, but about $40 less, though it is only Bronze 80+ rated, rather than Gold.
Barring some last minute suggestions, I think I have all my parts picked and will order later this evening.