Technically, it can be done, but see my edited post again.
Also, it's quite interesting that themes have a portrait lock screen image/icon, and the plugin has:
_HILDON_WM_ACTION_NO_TRANSITIONS
_HILDON_PORTRAIT_MODE_SUPPORT
_HILDON_PORTRAIT_MODE_REQUEST
set on it, so it should rotate in portrait mode, yet I don't know why it doesn't.

Edit: ha, it flips to some inverted state when forced to with CTRL+SHIFT+R, so you have to hold the phone upside down in portrait mode to see it properly, so I guess it was disabled on purpose.