So over a year has passed since this post has had any editing.
I recently lost the power supply on my 2009 iMac 27" BTO with i7 CPU.
I had ordered the parts mentioned in another internal dual hard drive upgrade article based on the 2010 iMac that DOES have the third port.
Once in the iMac, I ended up disconnecting the DVD drive Combined Power and Data cable. I used the purchased SATA dual drive power cable to power up the SDD, the and purchased Data cable plugged into the DVD Data connector and installed the SSD in the purchased replacement "Wall" component that Apple used for their dual-drive 2010 iMacs.
My motherboard also has the pads for another SATA Data connector and resistors exactly like the main Hard Drive SATA Port. I, too also considered finding the connector and chip resistors and soldering up the port.
The ultimate question is whether SATA follows the architecture of ATA where the basic controller could manage two drives, one being "Master" while the other being "Slave". With ATA, the Data was on a parallel ribbon cable and the drives needed address jumpers to work properly. With SATA, the Data lines are not shared meaning if SATA controllers intrinsically can handle two devices, it only means adding the second device and the chipset will "take care of the rest" (were it ONLY that easy). The answer lies within the details of the "5 Series" Chipset.
The "About this Mac" SATA Device Tree shows two independent chipsets. Under the chipsets, the drives are disk0s1, and disk1s1. A check of "About this Mac" on a later iMac with the third drive installed should tell definitively whether there is a third chipset or a second port to the original Hard Drive chipset.
Best regards,
John Pratchios