I don't know if it's a bug or a feature. It happened by chance, if you pass a value (rx, ry, rz) to a hotspot that is not a number, then it behaves like a layer. Example
Code
<krpano>
<preview type="grid(cube,64,64,512,0xCCCCCC,0x444444,0x999999);" />
<view fov="100"/>
<hotspot name="test" type="container" distorted="true" enabled="true" edge="leftbottom" width="500" height="500" bg="true" bgcolor="0x000000" bgalpha="0.5" bgborder="1 0x00FF00" bgbordermode="inside" bgborderblend="true" bgcapture="true" onclick="set(rx, NaN);"/>
</krpano>