Alright, so as an exercise in over-engineering I may have come up with a solution. However, this solution is so overtly complex it's probably not even worth it. That said here's the solution I came up with.
First, I should note a couple of things left out of my initial post. In addition to my IPAC2 I also have an IPAC4 which apparently does support two separate shift keys (one shift key for P1 and P2 and another for P3 and P4). I'm also looking to use the PACLink adapter from Ultimarc for XInput while also having default behavior of sending keystrokes to PC. The PACLink is supposed to only work with the high speed expansion interface of the IPAC2 through use of a 5-pin connector. However, in this example I will be trying to make it work with the IPAC4 which provides two independent shift keys.
To make this work I'll also need some more components:
6-pin PS/2 Male to USB Female adapterhttp://cdn2.bigcommerce.com/server3300/hhh6t/products/282/images/1665/4xusbfps2m__31055.1380219421.300.300.jpg?c=2
USB Male to two USB Female splitterhttp://www.usbfirewire.com/images/Product/large/298.jpg?1491715059
5-pin Male to USB Male adapterhttps://images-na.ssl-images-amazon.com/images/I/21NdH39vtTL.jpg
Standard USB Male to USB Male cablehttp://www.uniprolaptimer.com/images/usb_to_usb_cable-p.png
Now, with this mess of wires and adapters I could theoretically piece it all together as such:
IPAC4 -> 6-pin M to USB F -> USB M to two USB Female Split -> PATH1/PATH2
PATH 1: (Note: this path is actually doubled with the use of another splitter to support two PACLinks)
USB M to 5-pin M -> PACLink -> PC
USB M to USB M Cable -> PC
Well, that's a pretty nasty looking but hopefully you get the idea.
With this setup, I should be able to route my P2 controls to the P3 switches on the IPAC4 thus getting me the separate shift key I so desperately need. After that headache it should just be configuration hell setting up all new .ipc profiles bearing in mind at all times that my P2 is really P3 as far as the IPAC is concerned.
I know it's nasty looking, which is why I may not go through with this option but I am curious if this would work out or if there is something I haven't taken into consideration.