OpenERP unique serial number

General information for the business: we sell stuff
Kind of development: Customization of existing program
Platform: OpenERP
Num. of modules: 1
Description of requirements/functionality: Modifications need to be made within a new module inheriting the existing openerp functionality. (to be able to update the original openerp system in the future without conflicting files)
2) Name the module Unique Serial Number
3) Inherit and extend the view & model stock.production.lot. additional
requirenments are:
a. Each new serial number must be unique, before storing the new serial
number, a query to the database must be made to ensure this serial number does not exist already. Could also be implemented in modifying the database to automatically check that this field needs to be unique
4) Inherit and extend mrp.production with these new additional requirenments:
a. When assigning a serial number to a produced product, only existing
numbers should be assigned to. There is a tickbox which allows that feature, please select the tickbox by default and make it readonly OR remove it when you’re able to implement this in the object by default.
b. Each serial number which gets assigned to a produced product needs tob e blocked for further assignments to other products. My customer wans to have a unique serial number for every product which can only exist one time. Please implement this on this object (mrp.production) or if it is easier implement this in the stock.production.lot object or please advise where it would make most sense of it.
c. When a production order is confirmed – then it is not possible to modify the picked products from the BOM anymore. We would like to have the feature, that even the production order is confirmed, that there is an edit button on the top which allows to add or remove products to the manufacturing order.
Extra notes: