There is no plugin system in Armory yet. It was in the talks earlier.

EDIT: There are alternatives to locking outputs, though.  You could track the transactions created and then prefer outputs that haven't ever been used in this way, and if that's not possible, prefer outputs that were used in older transactions to those used in newer transactions - and in the case where you do have to reuse outputs, issue a warning ("WARNING: This transactions conflicts with transaction xxx created on xx/xx/xxxx; you will not be able to broadcast both.")

That would mean that if you create several transactions then EITHER everything will just work OR you will be told that it's not going to work.

roy