I was at a Marlins game last year and they did put up a picture and a messag of congratulations to a couple for seeing all 30 MLB parks. But I am not sure if they had to pay to get it up there or what the logistics of it were. So, stadiums will do something, but I'm not sure how much. You could probably call customer relations for whichever stadium will be your 30th and see what they could do for you.
Now that I'm thinking about it, I recall seeing one a couple years ago too. One had pictures of them in front of the stadium with a banner and the Marlins mascot get put up on the Jumbotron. The other was just a standard block text message of congratulations that they also used to birthdays and anniverseries and other congratulatory messages.
It was actually the one I saw last year that made me want to do that for myself, and prompted me to join this site after finding it while doing a google search to find ballpark trip planning.
You're welcome. I've got a thread going over in the Travel section about the parks I'm hoping to get to this summer, so any info you've got to add would be welcome.
Ive been to 3 out of 4 of those (missing Arizona). Enjoy them. San Francisco's is my favorite out of those 4. Take the time to walk around the stadium. Great view of SF Bay from there too. Safeco Field is gorgeous but as is the case with retractable roof stadiums, it feels much too big and you can end up too far from the field. 2 great cities as well so hopefully you have time to enjoy them too. If you have questions about the cities, please feel free to post them and I'll try to help (SF will be easier than Seattle for me). The day I went to the Angels Stadium, it was 105 degrees so I sat in my seat for 30 seconds and ran for cover. Then lost my car in the parking lot (was a rental and I had just picked it up). Ok stadium but nothing spectacular.
I would agree that you ought to contact the teams about an announcement and do it sooner rather than later.
Congratulations on almost getting to 30. I think you will really enjoy San Francisco. It is the best in baseball in my opinion, neglecting historical nostalgia for places like Wrigley (I have seen 15 so far). I've been to many games in SF being from California and the team seems to recognize fan groups pretty frequently. As others have suggested, contact customer relations and tell them your story. I would not be surprised if they did something for you.