GNOME != Fedora

Oh yea, SSH_AGENT isn't defined in the cases when it doesn't work. Good luck tracing the guts of a modern desktop to find out why though. And ssh-agent is correctly at the very apex of the process tree for a session so variables are being culled from the environment for some reason that isn't documented anywhere. I still haven't found a way to reliably launch a .desktop file from a command line so debugging options are limited. xdg-open is what Google turns up and it almost works but the failure modes on it differ so much that it obviously isn't what is at the base of either GNOME or XFCE's launch action.

Hmm. Now that ya have me looking again at the guts of the thing, I think the clue is in /etc/xdg/xfce4/xinitrc. Looks like it wants to use gpg-agent to do everything. And I have both it and ssh-agent running so bad things are almost certain to be happening. Will have to go slowly over it until it makes sense and figure out how to kill one of them off.