Hi chucktodd,
some time passed since your last comment here but I've looked exactly for this topic (SS pin 53 as an output on Mega in communication by SPI e.g. the ethernet shield).
As I studied the datasheet of the ATmega2560 I found an interesting statement in table 21-1 (page 192 in the complete version of the datasheet).
It explains that it does NOT matter ("user defined"), if this pin is defined as in- or output in the MASTER-mode.
Are you sure this is an hardware-issue. As the hardware appearently accepts either data direction IMHO it only can be a software- (library-) issue. Maybe only one additional codeline is enough to define the Mega as master before starting every single communication.
I haven't even tried it yet but it seems to be able to be solved easily.
I am sure you have much more experience in the arduino- libraries, so I'd like to know your opinion if it is worth it to dig into their intestines and explore (and maybe fix) the reason for SPI not working with pin 53 as an input.
deschain