That's all well and good, but it would have made sense to silkscreen it on the board, add a second spi header, he'll, maybe even level shift the logic in the 5v lines. After all, unless there's a diode on miso or something else, you're asking to burn out the due with those shields. So mah e if it's write only you bought backwards compatibility.
Reguardless, do something stupid like put a 5v vcc next to 3.3v logic in the same header, you should at the very least silkscreen it on the board so people don't fry $1200 of hardware for write only backwards compatibility.