A lot of changes are going into the article metadata model for OJS 2.2 to incorporate better multilingual capabilities. This will be a stepping-stone towards extensible/modifiable metadata sets. At that point features like this will become much easier to add e.g. as plugins. If in the meantime this is a critical feature for you, and if you have access to a PHP developer, I can describe the changes required in detail; otherwise, I'd suggest waiting until the new metadata model, as changes made now would need to be re-implemented to take advantage of it.
I hope that answers your question!
Open Journal Systems Team
Don't miss the First International PKP Scholarly Publishing Conference
July 11 - 13, 2007, Vancouver, BC, Canada