Ooh, nice!
I made a lot of extension/adjustments/redesign to the custom animation system, and hopefully got it running as you intended:
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>Tail: Looking at the file, your FBX animation doesn't seem to attempt to use TailBase: only Tail1/Tail2/Tail3/Tail3_end - and those can't be used because they are placed differently from one horse to another - TailBase is the only bone that's common to everyone. Are you sure Rigify didn't just overlook TailBase or something like that?
>Facial bones/ears/tongue: you used "excludechildren:Head", which filter out all of that. Also the "Jaw" bone is named "Javv" in pony models, for some reason.
For the rest:
>Tongue: Forgot I was using an external tongue. Changed it so custom animations use the original SFM tongue instead.
>Jaw/Mouth/Tongue: default position for those can change drastically between models, and that messed the mouth offset in your anim. I uplodaed the exact models used in-game - hopefully using those as reference will reduce the problem:
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In the meantime, I added two new config commands:
"Animation-Ramage test anim-excludechildrenposition:Mouth"
"Animation-Ramage test anim-includeposition:Tongue"
They prevent everything parented under the mouth from being moved out of its default position, while letting the tongue do whatever. Not really elegant, but at least this way it work without having to redo the animation.
>Eyes: Added a new config command: "Animation-Ramage test anim-eyetracking: pelvis". You can use it to lock eyes to any existing bone. It seem a few bones like "LeftButt" and "RightButt" don't affect any meshes, so an animation could probably repurpose them as eye targets.
>The giant pony outside: animation file was baked with a low-poly mesh inside, which was loaded as is and standing outside, towering over the map. Added a new config parameter "Model-Ramage test anim-loading:0", so you can tell the system to ignore any model inside.
All of that will be available in the next update, hopefully next Monday if I can get Custom Characters to work properly by then.