I decided to email Chuck Westfall of Canon USA and he came back with this helpful response. It appears that I had forgotten the first rule of mastering any piece photographic equipment - "READ THE MANUAL"
"Hi, Derryck:
A note at the bottom of page 120 in the English edition of the EOS 5D Mark II Instruction Manual indicates that non-Canon flash units will not fire in Live View mode unless Silent Shooting is set to Disable. Please check your camera to ensure that you've adjusted this setting accordingly.
Thanks for reading Tech Tips!
Best Regards,
Chuck Westfall
Technical Advisor/Professional Products Marketing Division
Consumer Imaging Group/Canon U.S.A., Inc."