There is no plugin system in Armory yet. It was in the talks earlier.
Incidently, just minutes ago I asked about the status of that in another thread.. I think this would have a *lot* of potential.

In your case, you might create a solution with armoryd.py, or have someone program it. Should be possible with some python-web-magic which creates/polls the BitFinex address and triggers armoryd to do its part.

Ente