All you have to do is open the player.npc file which is located in the assets0.pk3 in the base folder under ext_data/npcs. In there it has the line "playermodel player" just change the "player" to the name of the model you want to appear in the cutsceens. Pack it back up and pop it into the base folder and that should work for ya. It will only change the character in the cutsceens so you'll still need to do a playermodel swap once in game.
DO NOT EDIT THE ASSETS0.PK3 !!!
If you do, you won't be able to join Pure Servers, and probarly most servers anyway, so don't edit that. !!
Anyway, i don't think that method works, though im not going to try it at all, i don't want to ruin my MP experience.