If anyone reads this, there are in fact two workarounds in Windows to open the buddy list from a global hotkey:

1) Set the Shortcut key property of the Explorer shortcut that loads Pidgin, to say ctrl-alt-B; this will open/restore the buddy list instantly (be it closed or minimised). If already focused, nothing happens. If the window is minimised, it is opened but not focused.

2) Use the following AutoHotkey script:

(your chosen shortcut here)::
IfWinActive, Buddy List
{
WinClose
return
}

IfWinExist, Buddy List
WinActivate
else
Run, C:\Program Files\Pidgin\pidgin.exe

return

The above:

* Closes the buddy list if it's focused
* Focuses the buddy list if it's open
* Opens the buddy list if it's closed

Obviously substitute in a different window title if it's localised. Not sure how you'd deal with a Unicode window title ...