You can easily make such subplugin on your own. All you need is to hook teamplay_round_start or arena_round_start event. In event callback you can do what you want (in your case, AcceptEntityInput named SetCustomModel with SetVariantString to your model path (hardcode or use FF2_GetAbilityArgumentString (or FF2_GetArgS on some forks))
There is a caveat, the player can change own model using plugins (if any).
In this case, you need to somehow hook prop named m_ModelName and force player to use your model while arena round preparing.