I’m flattered that you would think of me as some sort of “hero” (my ego doesn’t need any more help, thanks :D), but I’m having a little trouble coming up with something that might assist you. Partly, it’s that, in my work and art, I have some… connections, people you may recognize, so I know firsthand how jealously they try to guard their privacy from the daily onslaught of public attention. I’m a bit torn, therefore, about using what I know to help someone pierce that veil.
Further, and I think this was mentioned in at least one of the previous threads on this subject, many celebrities are leery about producing memorabilia like this for fear it’ll end up on eBay or that their well-intentioned efforts will be otherwise exploited and/or mocked. So whatever you’re asking for, it should be specific to your husband, but not so specific it’ll require Marino to sit down for half an hour to work on something for somebody he’s never met.
I do have an idea, though, that might work as a compromise. Since it’s unlikely that Marino would be willing to take the time to produce something like that, you have to make it as easy as possible for him. To wit: Why not buy a birthday card and send it with a request for a quick inscription? That’s certainly more personal than an autographed photo, and doesn’t really take a heck of a lot more time. And if your husband doesn’t know what you have planned, this would be a huge surprise almost as good as a letter.
Assuming the agent (or one of his assistants) saw the previous two letters, you could even present this final request as a compromise: “I’m growing resigned to the probability that my previous desire won’t be met, but I refuse to give up because of how much I know this’ll mean to my husband, so, given the likelihood that the letter thing is a non-starter, here’s something that won’t be nearly as much effort but that’ll be almost as good.” That might get a better response from his handlers.
Of course, if you really want to make the agent’s day, you could even write the letter that’s supposed to be from Marino yourself and send it along with a request for signature. In this scenario, the letter you write would go something like this: “Hi, this is Dan Marino. Your crazy wife has been writing and calling us every week for a year, trying to get me to write a letter to you, and we’re a little worried she’s going to handcuff herself to our office door, so we gave in and wrote this to you. Sincerely…” 
Obviously, it would have to be better than that (you don’t want them thinking you’d really chain yourself to something, because you’d risk a restraining order or something), but it could very well separate your request from the dozens of others that arrive daily.
Alternatively, to get their attention, you could hand-stencil your request on the side of a water buffalo, but that would be awfully expensive to ship. Too bad you can’t record a holographic request and send it in a droid: “Help me, Obi-Wan Marino. You’re my only hope.”
Seriously, I think sending a birthday card is probably your best bet here. Good luck.
