That is irrelevant, the whole point of yellow labels is to determine that a signature is authentic, regardless as not whether or not someone does specific cgc signing events.
Again, when a future incentive like this comes out (which there will be) where every single copy of that comic is signed then it should have a yellow label because there aren't any that don't have them.
Quite frankly, if the scenario outlined above results in a label that questions the authenticity of the signature I'll be switching my dealer account over to CBCS. And yeah, I'm just one person, but I know of plenty of other dealers who have qualms with several of CGC practices, especially the inflated scores that books receive when submitted and graded on-site at a conventions.