All plasmoid's could get an associated shortcut. However, Chat plasmoid doesnt open up when its shortcut is activated. This means to use the plasmoid you 'have' to use mouse.
Changing the Plasmoid to be a PopupApplet would make the main plasma global keyboard shortcut work (and popup the chat). Though we'd still then need key handling inside there to switch between chats.
It actually does have a hotkey, although it's currently hardcoded to Ctrl + Shift + I (or Ctrl + Shift + J, I don't remember). But that only opens a chat if it has unread messages.
Fixed with: http://git.reviewboard.kde.org/r/108897/