Short answer is no it would not work whether they scan it or not. The backend system is not in place to enable PreCheck.

In order for PreCheck to work, when checking in, airline has to make a request to TSA to determine if passenger is PreCheck eligible. If TSA deems passenger eligible, LH encodes boarding pass to reflect PreCheck eligible.

At a minimum, LH needs the following for PreCheck,
1.) Permission from TSA to be a PreCheck airline.
2.) Change existing process to make PreCheck query to TSA systems.
3.) Change boarding pass so that PreCheck eligibility is encoded in BP and "signed" the BP (and provide public key to TSA) to prevent forgery. I just scanned a mobile LH BP from past week. It is not signed.