Overriding Game Models
POSSIBLY OBSOLETE: Trying to replicate this in current versions of Source Filmmaker appears to fail for most models, including for custom player models that work fine in TF2.
You can change the player model used by the SFM to any model in your current mod paths. This is useful for recording game animations on other models, and works best on models similar in size and proportion that have (many of) the animations that the base player models have. If any game animations are missing from the new model, the game will put put the model in its default pose (generally the "T Pose") instead of applying the missing animation. For example, the robot scout is missing its swim animations, so as soon as you run into water as the robot scout, you'll see your model snap to the T pose. (assuming you're in thirdperson mode)
To change the player model used by the SFM in game mode, copy that player's script from usermod\scripts\playerclasses\examples up a directory to usermod\scripts\playerclasses, and change the "model" and "model_hwm" fields to your new model.