While Steam inventory isn’t a viable solution and a dev talked about it in another thread related, this is a long term mistake to keep it like that, with skins unavailable offline.
The most logical solution i can see here would be to use the online mode to write some files the offline mode will access which contain the owned mtx. That way the offline mode is still fully offline, and will just read files written while the game was accessed in online mode.
Also, sooner or later, it’d not be surprising the offline mode will have mods and any skin will be available there, including custom ones.
It somehow shows a bit more how conflictual making a GaaS (Game as a Service) and maintaining an offline mode is.