I had a quick look to see if anyone had brought this up before but my search-fu is probably weak.
The Anti-Paladin and the Paladin are the only characters that have to be of one specific alignment.
It seems that the Warlock and Zaharan Ruinguard can be of any alignment, unless I have missed something. They have a very chaotic flavour to them though. Do you prefer your Warlocks and Ruinguards to be of any alignment?
The Warlock class is described as being comprised of sinister individuals who seek alliances with dark beings. It has a very suitably dark, demonic feel to it. It does state that some become adventurers for heroic motives though.
The Zaharan Ruinguards are described as soldiers of a dead empire of chaos destroyed by the forces of Law. Now they use the knowledge of chaos to enhance their martial/magical skills. Some though are warriors dreaming of rebulding their empire or adventurers who have rejected chaos. This would be suitable for a lawfully aligned PC methinks.
They do have a reaction roll bonuses when encountering chaotic monsters who may remember ancient pacts. Plus any chaotic characters/monsters gain morale bonuses as well from the PC.
There is a nice Elric of Melnibone feel to this class. And IIRC Elric rejected The lords of Chaos and eventually fought against them. You could play a lawful Zaharan Ruinguard along these lines. It seems strange that they still derive their powers from Chaos, if they can be of lawful alignment though.
Of course you tailor the rules to fit your game but how do you play the Warlock and Ruinguard classes?