Since it seems fairly easy for Dixingians to get to Haixing (at least when Sha Ya and her sister do it), maybe there's a kind of smuggling ring that brings the latest Haixing fashions and implements to Dixing? And fashion magazines... somewhere in Dixing there's a guy whose special power it is to copy Haixing hair cuts.

I got the feeling that Haixing is what many ordinary Dixingren aspire to (or would, if they didn't know the treaty forbids it), so I could see that kind of thing happening. A bit like people in Soviet Russia wanting to buy your jeans! The people at the palace do dress very differently.