So, ummm...

See these traces on the board here?

See this other hole on the left?

Looks like this is where Apple's proprietary SSD connector should reside.

Placing the SSD from the 13" MacBook Pro Retina Display confirms this notion. If the shoe fits...

We purchased the "bargain-basement" version of the iMac. We're assuming that the more-expensive version—one that has the built-to-order Fusion drive option—has this connector soldered onto the board, and a 128GB SSD placed into said connector.

Looks like there'll be no love for our awesome dual-drive kit from this iMac generation. :(