You can reset the NVRAM with your keyboard?
Some keyboards, even some Apple keyboards does not work for pre-boot configuration support with a MacPro5,1, like this first generation 
Apple Keyboard with Numerical Keys made from August 2007 to sometime in 2008 that does not work for pre-boot configuration support with a MacPro5,1, the issue is the ProductId 
0x0220 with 
0.69 firmware:
	
	
	
		Code:
	
	
		Apple Keyboard:
  Product ID:    0x0220
  Vendor ID:    0x05ac (Apple Inc.)
  Version:     0.69
  Speed:    Up to 1.5 Mb/s
  Manufacturer:    Apple, Inc
  Location ID:    0xfd120000 / 4
  Current Available (mA):    100
  Current Required (mA):    20
	 
 
This second (or third?!?) generation with ProductID 
0x024f and firmware 
0.74 works perfectly:
	
	
	
		Code:
	
	
		Apple Keyboard:
  Product ID:    0x024f
  Vendor ID:    0x05ac (Apple Inc.)
  Version:    0.74
  Speed:    Up to 1.5 Mb/s
  Manufacturer:    Apple Inc.
  Location ID:    0xfa232000 / 4
  Current Available (mA):    500
  Current Required (mA):    20
  Extra Operating Current (mA):    0
	 
 
There are other keyboards made by Apple that have issues with pre-boot configuration support with a MacPro5,1 too, but AFAIK no one compiled a list.