After a lot of research this looks like a viable option: SSD Adaptor
Waiting on a response from them for compatibility with the Late 2013 iMac's. This should work as they have made very little changes to the logic board, however if examined closely the shielding on the pcie connector is sligthly different with a cut on the top (2012) model compared to a non cut shield on the 2013. I have ordered the kit regardless, and I will confirm everything once the kit and adhesive strips are in hand.
The only down side that I can see at this point is that power is sourced from the primary HD and is split (supplied splitter with kit) to accommodate the SSD addon. I will also take the time to do a video walkthrough, as it is sorely needed.
Update: All SSD Blade Kits designed for 2012 models are not compatible with the 2013 models!
The following link can shed some light on this problem: HERE
What can be done now?
My suggestion for now is a part pulled from late 2013 MacBook Pro's or iMacs will work for the SSD option, with a Homebrew Fusion setup a few terminal commands away. I would highly suggest skipping over ANY 128GB SSD modules as they are considerably slower than the 256GB modules. Please be aware that 2012-Early 2013 modules are not the same as Late 2013! The retaining pin (off center recess on the SSD module) on older modules are 1-2mm right of where the new retention resides. The contacts and number of pins are also different. Also as a side note a 256GB Samsung 840 Pro registered 38MB's write with a 130MB read rating through BlackMagic, using the normal SATA connection to the stock drive! I have a 15 day timeframe before my Samsung 256 SSD designed for the PCIe is delivered, at that time i can do further testing and provide a guide if necessary. A full guide can be done once a blade kit is finalized for this model.
Apple has indeed made it extremely difficult, but the war is not over!
I will be actively involved in getting more information from OWC and a possible fix through microsatacables for a revision on their 2012 SSD Blade Kit. (microsatacables.com also provided the blade kit for this test, many thanks to them for their overwhelming support for the modding community!) What you can do is contact OWC, tell them to provide us Late 2013 SSD modules for the PCIe.
I'll add to this once more information is available. You do still have the option of using a thunderbolt driven external drive, but the cost is still pretty high.