Agree there are some simple ways to do this. some points to consider
-If the calculation of the flat pattern follows the rule of the fabrication, remember that every manufacturer can obtain different dimensions according to the method of the fabrication (bending tables) (Kfactor) etc so I will encorage the use of Bending tables because it is base on the test direct with the workshop...etc
-Asuming everything is fine with the method all that you have to do is check where Inventor is keeping this information if is in 2 Parameters (lets say Width and Height) all you have do is
Make sure the rule is always running (go to manage, check the event triggers and copy the rule under every parameter change, etc. With this you will garantee that if anything change dimension wise, the rule will run to update the dimensions.
Now the dimension will be in a custom field of the Iproperties and you can passe it to the BOM, the Part List or map the properties in Vault