While it’s not really a solution to your problem, there is also the potential of user created mods which add in new, custom skins and content.
EHG isn’t really sure where they stand on mods, but from my query last year, they were open to mods in offline and stated they would neither aid nor hinder their creation and would monitor what becomes of such a scene, if one does appear.
Maybe a solution to MTX skins being unusable in offline would be to mod in your own skins, or creating a modding scene with some new transmog appearances to match your interests.
Source:
Mods are great. We don’t have any plans to directly support or hinder their creation. If an offline modding community emerges, it is possible that we might consider revisiting this in the future. It’s a pretty low priority topic for us at the moment.
I do agree that skins should be accessible in offline mode, but it’s probably an authentication nightmare since those skins have DRM attached since they’re only obtainable for real-life money.