JFZ90,
You are right to say that the a mis-routed strap could not move the handle while the seat pin was in. The sequence would be strap in - seat pin out - taxy - tighten straps. That's because it's usually possible to tighten the straps a little more once you've been sat down for a bit. Common times to do this might be pre-take off and before aerobatics (the S in HASELL checks = security). If your business is aeros, you'll want tight straps!
Alternatively, you might move your leg a lot after making the seat live (eg a full rudder check) and in doing so apply enough tension to the strap to move the handle.