Of all the pins I could have chosen to repurpose for the secondary SPI channel clock . . .

AREF SIGNAL HAS A SNEAKY CAPACITOR TO GROUND I DIDN'T NOTICE

I discovered alternative schematic files for the Arduino Mega 2560 which conflict with the ones provided by the official site. In particular, the signal AREF which is show on the official diagram as connected solely to the connector block pin labeled AREF, in the alternatives I find a capacitor on that signal with the other end tied to ground. 

Not a good thing to have on a multi megahertz clock line. At 4 MHz this capacitor has an impedance of 0.4 ohms, essentially a dead short to the oscillator signal. Even if my setup of the secondary SPI channel on USART 2 is correct, I wouldn't have much of a clock signal available. 

REMOVING THE CAPACITOR FROM MY BOARD

Reworking to pull that capacitor off the board was quick and easy once I knew it had to be done. If by chance the USART clock was damaged by the short, I have three other USARTs I can switch over to on this board.