'A note "Shall meet IPC Class x ,blah blah, except where precluded by original design" doesn't sound like the correct approach to me - it could unintentionally result in the designer not getting vital feedback from the fabricator and assembler.'

James makes a good point.  It also makes it easy for the assembler to say "I couldn't solder it right, it's precluded by your design".  Much better to add "Except as noted", then make sure it's noted.  Makes for better design discipline and is more likely to get a response from manufacturing if you accidentally missed something.

Pete