I just installed one of the metal bar locks from HD or LWS and it was
able to be installed with the sliding door on the outside. It also was
setup to lock in a venting position.
good luck

Steve wrote:
My sister has she would like to lock partially open - a typical glass
sliding patio door - except it appears to have been installed
backwards! The sliding porting of the door is on the inside of the
house. The fixed portion is outside. She would like to be able to
'lock' it open a couple of inches for air flow. If the sliding portion
was outside, a simple locking bar against it and the fixed portion
would work, but as it is reversed, the bar would be on the outside. I
thought about putting a locking pin in the top or bottom, but there is
not enough clearance to drill the frame with out hitting the glass. I
suggested a door lock chain, but she doesn't like that idea.

If I was her big brother, I'm sure I could figure it out, but alas, I'm
her younger brother...