I think it could be done by swapping the 3D models in the Unity Assets Viewer (exporting the raw files and re-importing them with another path). However, there are tons of files involved in 3D models in the assets so I'm not sure that it's an efficient method.

Trying to replace the model ID (or name) in the source code is not so simple either...
(Mainly because the animations of the trance models are the animations of the default model and thus the default model should be loaded first)

So, it seems that the easiest and safest way to do that is to call the function "btl_vfx::SetTranceModel" at the right moment. Since it is still a bit complicate, here is a modded file:
https://www.dropbox.com/s/ia1epc8iodezg63/PC_AlwaysTranceMod.zip?dl=1
There's a .hws as well but I don't guarantee it to work (you need to have the exact same version of the game as me which is normally the latest update, but there might be some tweaks).

That worked pretty well (but unfortunately it makes everyone their trance outfits) unfortunately I'm playing with the Alternate Fantasy mod so I had to put back my back up of that dll