>Was copyright the only reason why Hal appear as a cameo?
Yeah. DC just couldn't get the rights from themselves. Paul Dini would write a very angry and well-worded letter to Paul Dini, have it mailed back to DC Comics, read it, and tear it up in frustration for Paul Dini even considering using Hal Jordan for copyright reasons. Dan DiDio described the ordeal as "pretty fucking confusing."
Joking aside, Hal Jordan was off the table completely. When the show started production he was already labeled as an irredeemable piece of shit that tried killing everyone during Zero Hour.
Kyle Rayner would have been a good candidate, but John Stewart had 24 years of publication on him. Guy Gardner was being tossed around between different runs because nobody knew how to write for him.
He was pretty much picked for his popularity (at the time) and because Bruce Timm and friends didn't want to get angry letters about there being no diversity in their cartoon.
Take it for what you will, but I think they did a decent job with his character. Not so much with the constructs he made as they were pretty out-of-character for the first two seasons, but otherwise decent.