I just got a message back from Avid support. If I understand them correctly MC will only export and read quicktime AVC-Intra with a Nitris DX:
“AVC-Intra only supported with Nitris Dx and that is the main reason for which under the system with Mojo SDI the export fails. ”
I also noticed that when I open up those AVC-Intra quicktimes I exported, the codec is listed as Avid AVC-I, which would support the idea that the MC with Nitris has its own flavor of AVC-Intra codec. And from Avid’s AVC-I Codec Module FAQ:
Will Media Composer Nitris DX include the AVC-Intra encoder?
Composer will ship only with the NitrisDX Codec, but can get the AVC-Intra module as option. Symphony systems will ship with the AVC-Intra included.