I'm looking for a guideline (Z540, ISO 17025 etc.) that says whether or not you may extend the calibration of a standard after it has gone past its due date, and if so how long.
that is up to your quality manual
You might want to do this only if you have enough objective historical evidence about the stability/accuracy of the standard. This is used to support the decision to take the risk that extending the cal interval will result in an in-tolerance condition when the standard is re-calibrated.
Extending cal intervals is all about risk management. What would be the ramifications if the standard was found to be out of tolerance at its next re-calibration?