It wont truly work as you think. Not all the models are divided in parts.
If the plume isnt attached as a preexisting accesory, but rather already fused with the model, how are you gonna make it dissapear, without touching the rest?
All I can think of is specific retextures for each case, with transparency in the back. But accesories arent retextures.
@Gingermace - personal preference, as with all accessories.
@Juances - I have to agree with you given my lack of understanding in the modeling. However, OOO dev'd up a way to add pieces and parts to models, I'm confident they could redo the current models as to open up the original possibility. Or as you said, the new accessory (a plumeless helm) is actually a new model in and of itself.
As such, I would trade in my helm + crowns for one without a plume.
The problem is that any helm that already has a plume on it just won't look right without it. Except maybe mighty cobalt.