After doing a general search both for online as well as high scores, i can only conclude i can't understand how it works easily with only 10 minutes of looking.
Oh i found interesting contents in the lua files, where the snapshots are taken, actually getting online, and where the high scores are registered. But i don't see where the online/offline are separate, rather it seems the character is registered when you create a new one and you go online.
This is probably the closest you'll get unless you want to pour over a lot more files than me. This particular section is for creation of a new character, and at the very end you find this tidbit...
tome-1.2.5.team/mod/class/Game.lua
-- Register the character online if possible
self.player:getUUID()
self:updateCurrentChar()
I'll give it a try myself since i make characters all the time, commenting these two lines out. Good case is nothing happens because i don't play online. Best case, it works for you, Worst case it breaks something....
edit: Well it didn't break anything :P Not for a new character anyways...