I have received letters asking permission, and they don't get consideration. I vote for the personal approach - dress conservatively, don't interrupt any work going on, tell him a 20 second bio on how you (a stranger) are NOT the slob hunter this farmer has dealt with in the past (afterall, there you are there asking permission),limit the scope of permission (yourself only would be a good start or you would hunt only on weekdays - get the idea?), and offer to do work will help your case. Seeing is better than reading, imho...
-fsh