Proposal: Add something like this to the spec:

   ]] Conformance checkers must allow users to separate pages for which @alt conformance check was performed, from pages (with the generator string) where it was not performed. [[

(The wording inspired from http://www.w3.org/TR/html5/obsolete#warnings-for-obsolete-but-conforming-features )

   Use case - in 3 steps:

1 Author runs the following four documents through the validator:
   doc A  with machine-checkable @alt errors + no meta generator string
   doc B w/error free machine-checkable @alt + no meta generator string
   doc C  with machine-checkable @alt errors + meta generator string
   doc D w/error free machine-checkable @alt + meta generator string
2 Validation results:
   doc A: non-conforming due to @alt conformance error;
   doc B: conforming;
   doc C: conforming;
   doc D: conforming;
3 Author's response to the validation results: 
   doc A: author fixes doc, validates anew and publishes;
   doc B, C and D: author is satisfied and publishes right away;

   Problems revealed in this use case: 

 FIRSTLY, for document C and D, the validator fails to inform the author
          that the docs were subject to a less thorough @alt conformance
          check than document A and B. This means that the author is
          is right to believe that B/C/D are equally conformant and 
          equally checked.
SECONDLY, HTML5 is actually clear that doc D, is non-conforming, quote:
       ]] (This case does not represent a case where the document is
          conforming, only that the generator could not determine
          appropriate alternative text [[

          Of course: That doc D is non-conforming, is not something that cannot be told, as long as the meta generator "exception" is in place. But it is the more important to give the user opportunity to understand that the reason for the lack of an error message, *might* simply be that the particualar issue was not checked for conformance.