Go to a different area (he'll likely show up) that requires the game to reset (like using the abyss scroll).

Neither seems to work, his voice appears in cutscenes but he just isn't there no matter how much I progress and using Walk the Abyss doesn't make him appear either.

Any other ideas?

No; I've had coterie members disappear and that always got them back for me - it even works if you tell them to stay (like in the clock since they have the bad habit of standing in the sunlight). You let them behind, go to the next area and there they are, safe and sound. I'd just start trying things that don't deem related like changing the lead character (perhaps TO Wilhelm?) before I panic. I'm really hoping you don't need to start a new game and/or reload the game to fix this.

I'm playing AoR 2011 if I didn't emphasize that enough-

It's pretty much the campaign in multiplayer format, I don't actually control the other party members and now that they're gone I know of no way to bring them back and it's making some fights (Othelio, anyone who wants to suck my blood) rather irritating.

Sorry - forgot that. Unfortunately, I only played the AoR2009 version as I couldn't get 2010 to work. Been wanting to try 2011 but haven't yet. Bottom line: I'm useless to help with this problem. The maker of AoR is active at planet Vampire and might be able to help.

Ah, ok. I'll check out Planet Vampire, thanks for the reference.

http://planet-vampire.com/forum/

There's a section for "Redemption". The modder's "name" is Javokis.