XDI message for retrieving one's private key?

Let's say =markus (=!91F2.8153.F600.AE24) would like to retrieve his private key from his XDI endpoint..

Would anyone have time to tell me what the XDI message for that would look like?

I.e. the actual serialized XDI data that goes over the wire.

I have an idea of how it would work, but I'd rather hear it from soneone else :)

Markus