What are the rules you are refering ?

I think the problem is, that the syandana goes over the shoulder, which would lead to clipping issues.
Looks good, on the more humanoid frames (like Banshee, Excal, Mag etc), but with the bulkier frames (Rhino, Chroma, Atlas etc), the syandana will clip through their shoulder.
The syandana looks great BTW.

they (DE) can change the positioning (and even size) of each syandana, which they alter all the time in hotfix notes, to properly lay on a warframe as intended. it's not uncommon for them to do so, even with their own syandana.

Who the ♥♥♥♥ cares? It's up to DE and if you want to get into rules, just look at Graxx

You want to evade rules? Make something of quality

Haha there actually are no official rules on something like this. Just that historically, they avoid stuff that goes over the shoulders and neck since they would be a nightmare to adjust for clipping. Now I don't know for sure, but I gave this a generous arc and made it cover only one shoulder in the hopes that it could be positioned to work on most of the frames.

would be an awesome accessory for the operator

I'm with Tuxedo :D
Give it to the operator ^_^

I think the potato kids already have enough over the shoulder stuff.

In fact, there already are syandana going on the front of the Warframe, the Uru syandana and the imperator syandana both have small bits wrapping around the warframe's neck, thus, I don't see why this syandana couldn't be put inside the game

maybe you could replace (or work around) the horn on her left shoulder with the syadana (as it only goes with banshee and banshee prime), that way it doesnt have that many compatibility issues with other warframes, and it will be more likely that DE will pick it up

perhaps replace the horn-over the shoulder part with a less standout part, and make it simple... but just a suggestion