It appears that it is possible to insert admin scripts into non FE games. Some, like Epix (https://www.roblox.com/Epix-Incorporated-Server-Suite-EISS-item?id=145629584), have a loadstring implemented as a lua vm running inside of lua.
The game owner must own the model.
Filtering enabled games are not affected.
Also, some original model author's accounts might have been compromised in the past. This means that compromised account can be used to update the model. From there it can be inserted into the game as you still own it.