The suggested solution:
run dconf-editor open desktop -> unity -> panel set systray-whitelist to "['all']"
doesn't work for nouveau.
Where I have nvidia propetriary driver icon appeared normally...
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You can't find desktop -> unity -> panel in ubuntu 12.10 in dconf-editor,
To add pidgin chat menu:
Open dconf-editor from terminal.
Goto com -> canonical -> indicator -> messages
In applications append the value 'pidgin.desktop', please don't replace the older values.
By doing above actions, pidgin will be added to chat menu, but it won't started automatically, to start automatically
Open 'startup applications' from dash.
In command type 'pidgin' (remove quotes and type in small letters) and in name and comment type anything as you like.
Close all apps and restart your computer (log out and log on again its enough).
You can find Pidgin now in chat menu and its automatically opened.
By doing this, it won't affect empathy and gwibber processes, if you don't need these apps uninstall in ubuntu software center.
Stay connected and happy chatting...
Pidgin won't hook to the messaging tray icon in 12.10 due to this bug. As a workaround until that's fixed, something similar to what the OP describes as “the suggested solution” will work; the path you want is com.canonical.Unity.Panel.systray-whitelist. The problem is this uses Pidgin's normal tray icons, which don't fit the Ubuntu colour standard... but I can live with that for a few days.
UPDATE Aug 2013: This is fixed in 13.04, now it hooks into the messaging icon again.