I checked my "Dictionary of Basic Japanese Grammar", in which Vてやる is incorporated into the item for Vてあげる (as Vてやる is used if one is of a higher position). In all example sentences they provide for this を is the particle used.
Also crosschecking here: https://jlptsensei.com/learn-j... None of the てやる example sentences use に (but rather を, も, or no particle at all).
Of course as I might just be overlooking something as well, but what I found so far supports how it is presented here on renshuu.
