1) What's a good way (following GEP of course) to deal with the 5V / 3V3 difference?
What is "GEP"?
As for what is supplied by the HC11, the Netduino (by means the MCU) is 5V tolerant so there's no problem.
As for what is supplied by the Netduino (3.3V), the "HC" voltage levels (68HC11) are able to recognize the logic high at 3V: again, no problem.
2) Is it true that the nd+2 can't operate as an SPI slave?
Yes, that's true. You can only try using the Netduino as master and the HC11 as slave.
3) Anyone know of a similar project that I can reference?
I'm not aware about similar projects, but (under the 2' answer context) should be pretty similar as you connect an Arduino.
Question: why don't you use the UART? It's much more convenient if you want exchange data between MCUs...
Biggest fault of Netduino? It runs by electricity.