Well, it works well, but there are some problems. You can't aim a gun left or right, you can't move and if you try to crouch while moving, the game CTD's, but aside from that, it only needs a good animation. The displace_position flag and extra lines only activate once you release the key, and you end up floating with the mid-jump animation, so i put the one at the start of the jump and it's like this. It helps getting behind cover.
If anyone wants it, the module_animations line is this: ["crouch", acf_enforce_lowerbody|acf_displace_position,
[0.3, "jump", 22, 22, arf_blend_in_1|arf_cyclic, 0, (0, 0, 0), 0],
],
and the module_mission_templates line is this: (0, 0, 0, [(key_is_down, key_x),],
[(get_player_agent_no,":agent"),
(agent_get_horse,":horse",":agent"),
(neg|gt,":horse",0),
(agent_set_animation,":agent","anim_crouch"),
]),