So if I understand correctly you want to change the appearance of the default polymorphs, after a character activates an item in its inventory.
The main problem will be finding out when the polymorph effect ends. Remember the players can end them at will and you have no way of detecting that, unless you use a heartbeat. Plus there are other, more severe issues. Say you change the appearance from a dynamic (e.g. human) to a non-dynamic type (say a dragon) with SetAppearanceType. The problem arises when you change it back to human: The character will have lost all customizations like head model, hair color, etc. So you would need to save all those values prior to changing the appearance and restore them when ending the polymorph and again it's kinda hard to detect when that happens.
It's possible to do it, but it will get very very messy. Maybe with a heartbeat script.
I recommend doing one the following instead:
1. Using the item directly
Using the item directly to apply a polymorph. You can then set the polymorph effect to locked and give it an infinite duration, which means the players can't end it themselves.
Upon activation you can save the head model and colors, then you polymorph and then you change the appearance. They could only end the polymorph effect by activating the item again, which solves the problem of not being able to detect when the effect ends. Then you can restore the appearance to the default polymorph one, then remove the polymorph effect and then restore the appearance (head model, hairs colors, etc) from the saved values.
Advantage: No 2da editing needed.
Disadvantage: A lot of scripting. Can be unsafe if not done properly = players can loose the appearance and colours, i.e. back to head #1, blonde hair and pale skin.
2 Custom hak with a modified polymorph.2da
Using a custom hak with a modified polymorph.2da. Add new polymorphs to it and then edit the spell scripts for wild shape and the polymorph spell (NW_S2_WildShape, NW_S2_ElemShape, X2_S2_GWildShp and NW_S0_ShapeChg).
Activating the item could set a variable on the character with a new polymorph ID from the modified 2da. When he uses one of the feats/spells you check for the variable and change the polymorph ID to the new one. Activating the item again deletes or overwrites the variable.
Advantage: Much safer. Players can end the polymorph at will from the menu. No need to save appearances and colours.
Disadvantage: You need to start using haks (if you're not already using them)
If haks are an option I would suggest using option 2. It's easier, safer and more comfortable for the players.