Don't know if you've used
http://www.sportscardforum.com/ttm.php, but they have a remarkable database of addresses there, so I just used the ones that have been successful for others from that site.
The other key to HOFers from what I've seen is including a "donation". I think I sent Dawson $30 to his foundation. I sent Ryno $20.
Also, I send a nice handwritten note. No clue if that helps or not, but most folks seem to read it, as everyone has provided inscriptions or whatever when I've asked.
Dawson is easy, in a way. As long as you include the cash/check for his foundation, he'll sign anything. Ryno was more a shot in the dark as I understand he can be spotty TTM. I've also heard he's amazing about signing in person.
The best part for me has been reminiscing while writing the notes to the players and coming home to see those packages in the mail.