Sorry to be slow in responding, but I cannot replicate this problem. SU's Install Extension option works as expected on my Mac which is running the latest version of Mountain Lion.
Dear Max,
Clever and instructive explanation.
And certainly a very useful tool. (i have fought some time ago with that spiral shape and haven't found in this time such a tool
http://i274.photobucket.com/albums/jj245/Spendauballet/SketchUp/horseshoestair_Final5.jpg
)