I feel like I should add the reverse_grip tag to this post but its wiki just talks about knives/bladed weapons... any similar such tags for firearms? I imagine it's a rare enough phenomenon that we haven't needed to bother with any.

I don't think it's necessary considering that death_the_kid + reverse_grip gives zero results and he always holds them like that. It's probably a Soul Eater ref, but I don't think it warrants the tag though,