There are some different answers to this:
First, maybe it could be coordinated with this initiative?
viewtopic.php?f=4&t=11253&hilit=shareConcerning an official and endorsed exchange site, the MH crew has (a while ago) taken the active stance that we don't want to manage such a site. This is because of various reasons including lack of resources, the spam problem and possible legal issues if copyrighted material gets posted.
However, if someone else is willing to go through the bother and handle these things, then we'll applaud it. So no: there's nothing stopping you from opening up a site which shares third-part makehuman assets (under whichever license you want), nor core makehuman assets (as long as you post those specifically under AGPL).
Anyway, if there's a target you feel is missing, maybe this is a good start?
https://www.youtube.com/watch?v=X13k7H3dNHoAnd finally, a tip: Since targets are plain text files, something as simple as a github repo would be perfectly fine for sharing them.