I actually believe that we can see an artifact of Lucas' changing casting ideas for the role of Luke in the ANH script drafts.
The third draft, written in August 1975, before any serious casting sessions started, introduces Luke Starkiller thusly:
LUKE STARKILLER, a farm-boy with heroic aspirations; who looks much younger than his twenty years. His short hair and baggy tunic give him the air of a simple-minded, but lovable lout with a prize-winning smile.
The "short hair" described here is rather generic, but perhaps recalls the clean-cut hairstyle of Flash Gordon. More importantly, it suggests that Lucas didn't have any specific ideas about casting yet.
The fourth draft script was completed on January 1, 1976. Lucas probably wrote the bulk of it during December. That was the month of the casting sessions; Seltzer auditioned early on, whereas Hamill came in at the tail end of the month.
It is thus probable that, if he worked his way through the script over the month, revising it scene by scene, Lucas' description of Luke (coming early in the text) might reflect a mind's-eye picture of Seltzer rather than Hamill.
The fourth draft text bears this out:
LUKE STARKILLER, a farm boy with heroic aspirations who looks much younger than his twenty-two years. His curly hair and baggy tunic give him the air of a simple, but lovable lad with a prize-winning smile.
Luke has suddenly gone up in age! (Seltzer was a few years older than Hamill, and likely older than Lucas originally had in mind; Luke was just 18 in the second draft.) Also, he now has Seltzer's "curly" (brown) hair instead of a WASPy crewcut.
Now check out the text of the revised fourth draft, from March 1976, by which time Hamill had been cast:
LUKE STARKILLER, a farm boy with heroic aspirations who looks much younger than his twenty years. His shaggy hair and baggy tunic give him the air of a simple, but lovable lad with a prize-winning smile.
Luke has gone down again in age, a fitting change if the younger Hamill had edged out Seltzer in Lucas' mind. What's more, Luke now visibly has Hamill's "shaggy" (blond) hair. Funny, that.