Have you considered pedals from flight sim controllers? I'm not sure how you would go about mapping them (I would assume they could just have a key bind mapped), but they could easily function as modifiers.

I saw you said that the orbweaver didn't suit you - which part? I bought a Logitech G13 and hated it (so much that I didn't use it for more than an hour) until on afternoon I sat down and really worked on it. Now I won't use anything else fore MMOs.