But yesterday I found this, which aside from showing some nice clips of the first movie, had Nakashima mentioning that when he and the director first talked together, they both said they wanted to make a manly anime like "Takarajima" or "Ashita no Joe" by Osamu Dezaki. Which made me immediately go, "OH, DUH! Osamu Dezaki!"
So yeah. Takarajima is an animated adaptation of Robert Louis Stevenson's Treasure Island, made in the late 70's and featuring a particularly badass Long John Silver.
Again, on hindsight this should have been obvious. But then I'm not exactly an expert on classic anime. :/ Anyway, now that I knew the reference, the similarities were pretty clear.
On top of that, I finally realized what interview I saw it mentioned in:
WEB Anime Style Interview for Episode 27
--Did the director have a rough sketch for that adult Simon?
Imaishi: No, I don't think so.
--So did you tell Nishigori-san "do it like this" in words?
Imaishi: Well, I guess you could say that.
--How did you want it done?
Imaishi: Uh, I said "make him like John Silver." (laugh)
--And with the way Nishigori-san interpreted it, that's how it turned out.
Imaishi: That's right. I also thought "This is it!" He properly watched all of "Takarajima" while we were making "Gurren Lagann" too.
Imaishi: So he understood what was great about it. There were no designs for Part C. That part was entirely drawn by Nishigori.
--Just what hardships did Simon endure for his chin to turn out like that? (laugh)
Imaishi: Well, I'm sure he went through a lot.
So it's official. Just as 4th arc Simon is a homage to Captain Harlock, Epilogue Simon is a homage to Long John Silver. I guess this explains why his chin turned out like that. XD