2.5in drives aren't a problem, as I have those devices that go in the back of the case and you can slide a 2.5in drive into.  As I'm using few enough cards, they (will) work well enough for when I convert my setup to ESXi.  It's finding a place for a 3.5in drive that may prove to be challenging.

 

Assuming you're referring to the expansion slot racks for 2.5" drives, I presume you know that you can get dual-drive racks as well:  http://www.moddiy.com/products/PCI-Expansion-Slot-to-Dual-2.5%22-IDE%7B47%7DSATA%7B47%7DSSD-Adapter-Hard-Drive-Rack-(2-x-SSD).html?gclid=CMu5n6_ApbcCFeLm7AodplwAxQ

 

With all the unused slots in the 4224, I'm sure these will hold more SSDs than you're likely to need 

 

As for the 3.5" drive, I'm at a loss as to why you don't want to simply use 4 motherboard ports to drive ONE of the breakout connectors on the drive rack.    This would give you 4 drives tied to motherboard ports -- not only satisfying your desire to use a motherboard port for parity;  but also letting you have 3 other drives of your choice on the hopefully-faster motherboard ports.    ... all with NO modifications to the case 

 

Reason is the rest of the drive bays in the norco unit are already full (or will be full shortly), and the rest of the ports on the motherboard I am dedicating to 2.5in SSD drives that will be used for an ESXi setup in the future.

Reason is the rest of the drive bays in the norco unit are already full (or will be full shortly), and the rest of the ports on the motherboard I am dedicating to 2.5in SSD drives that will be used for an ESXi setup in the future.

 

Makes sense.  HOWEVER ... it also means there's no reason to do what you're thinking about, since you can't use a motherboard port anyway if you switch UnRAID to an ESXi virtual machine, which will require that you pass-through the RAID controller.