Well, my first thought is that it's not a code he wrote. Maybe it's a code he was working on, given his own interests. Someone apparently wanted to kill this guy, knowing his interests they came up with some complete jargon, sent it has way, and then arranged to meet him so they could teach him the solution. If they sent the code to him via computer or something that might explain why he jotted it down to bring with him for referance.
That of course doesn't actually help in solving the mystery, of why he was killed, but it is a thought. If I had it in for this guy and knew his interests that might be how I'd bait a trap for him if I wanted to lure him to a location to meet someone he only knew via BBS systems or whatever.
It's also possible given the time period that this was going on is that it's a keystroke record for something. If he was playing older games even for the time, he might have been dealing with a solve that simply told you how to move in the game. The ( ) brackets being a way of seperating each sequence. Of course I can't think of any game that would look quite like that off the top of my head. The thing to consider is that some games like say "Ultima IV" considered how many keystrokes you entered or "moves" before you beat the game, and gave a running count when you saved. A few nerds would sort of compete to see how few keystrokes they could beat some of these games with (I'm pretty sure this isn't Ultima IV) and even post formulas for them.
Any other ideas I have would just be plain wierd and useless to the situation at hand. To summarize them though, if they haven't already they might want to have some occultists look at it or whatever.
I'm horrible at codes, but it occurs to me that if all of these experts couldn't break that code, including presumably the guy who did that Kryptos statue if they ever asked him, maybe it's not a code.
It's probably far too late, but they might want to do something like see if they have a list of what video games he was playing. I can't think of anything that would make sense in connection to, but it might "click" if they spotted the right one.... as useless to finding his killer as that would be (again).