I'm wondering if it would be allowed to have CWs made that are for specific Subeta items and would not work without that item.

For example, the exact reason I'm asking, would it be allowed to have items commissioned that specifically fit around the face tentacles of the cursed doubloons?

I believe it, in some way, has to work with the avatar. It is a little hard to say though, as I don't know what you're thinking of more specifically. For instance, I've done skeleton limbs intended to be worn with the Calaca set, but they 100% work with the normal (or any) HA skin. Likewise, I had to alter my mermaid background because it could be abused and used as an avatar replacement with items drawn specifically to be worn with it and not the avatar itself. When backgrounds had to have the HA cut out I was also told I would not be allowed to do zombie skin cutouts for them (I wanted my pastryland BG to have a Fantine's Kiss cutout version). So it's a little rocky, but also this doesn't sound like something that will be used in a way without the avatar. ... This... Was not helpful. I am no help, sorry.

I really wish I were an artist so I can just sketch my question lol

So, you know all of the little cw sweaters with like the hand touching the chin or something... they only sort of work with the squid head. I was wondering if it would allowed to do something that is essentially ONLY going to work WITH another subeta item.

It might be workable, but the artist would have to be very strategic so that the limbs/etc don't look like they're just floating around the HA when the cursed doubloon isn't there. Perhaps something that's off-shoulder/falling off. It can be styled specifically for the cursed doubloons (even the skin color !), but can work on normal bases still.