Polymorph.2da is a server side 2da - so I don't think the clients need to have it.
You can just drop it in the override on the server, and not have to distribute it to the players.
Other than that - I think hooking polymorph is do-able, but might be more trouble than its worth.
Think you would need to recreate the entire method, to check for locals etc, instead of reading from the 2da - as the 2DA contains pretty much everything:
Base Attributes, Skin resrefs, creature resrefs etc
It might be easier using the 2DA Approach