It turns out you can get this to work if you do.
Code:
UIPanelWindows["WorldMapFrame"] = nil
WorldMapFrame:SetAttribute("UIPanelLayout-area", nil)
WorldMapFrame:SetAttribute("UIPanelLayout-enabled", false)
tinsert(UISpecialFrames, "WorldMapFrame")
But then you sacrifice the effect of the map automatically pushing other frames (like CharacterFrame as pointed out by sezz) to the right. Mapster does not mind this as it makes the map frame moveable anyway.