lol nice one, I had a similar idea a way back with a different champion, coincidently, i have dread locks :D Rengar is already kinda rasta if you ask me, he does have dreads or braids of some sort, but I see where your going with this soo... maybe a crown and rasta symbols with rengar dressed in red, gold and green. Really can't see a successful "Rastadin" skin but i may be wrong, a beenie would work.
Personally, I'd like to see a rasta Yasuo :) they don't have to change his skin tone just give him dreads and a rastafari get up would be nice maybe have him smoking a blunt :3 Also a rasta Shen (if they can pull it off) simply because i love shen xD. So thats it, a rasta samurai and rasta ninja is what i wanna see in lol. They already are some dreaded skins in league like Headhunter Nidalee and Striker Lucian that fit with he theme well, but i dont think Riot would ever blatantly put Rasta in the title, even if the theme strongly portrays it.