I can't access it. I can't find it, at all. Vgui_drawtree spits out a fat lie that gives me nothing. There's no party chat in mainmenuoverride.res On top of that, huds.tf is empty and nobody responds if you ask something.
I think trying to change the font of the party chat stands equal to changing the normal chat. The only thing that differences it is the way a chat is accessed by others in game, i.e. your Team chat cant be seen by the other team and your Party chat cant be seen by anybody outside your party.