I've done this several times - 1st Howard Stark, 1st Giganta were both me in addition to a few others. The one time they didn't comply I figure it was simply overlooked rather then the result of some disagreement regarding my requested notation.
As previously suggested, mark it on your submission form if there is a note field (can't picture the form off-hand). You could also send a letter with the submission like this example I once sent. Doesn't hurt to call & make sure they've done it when you see your status go to Graded.
September 19, 2012
CGC Submission Center
PO Box 4738
Sarasota FL 34230
Dear Receiving Manager,
On the new label for the enclosed Eternals #7, could you please note the first appearances of Jemiah the Analyzer and Tefral the Surveyor?
There are several online sources that reference this issue as the first appearance of these 2 characters, ComicVine is an example.
Sincerely,
xxx
The way I see it, accurate notations are part of what we are paying for. They should honor the request if it's correct. I wouldn't be shy about requesting it any time of the year.