How to add a start repeat barline after a mid-staff key signature change

Select the measure in which the key change occurs and insert a blank measure in front of it;

Right-click on the measure you have just inserted and select Measure Properties…. In the Measure Duration section, change Actual to 1/64. Click OK to exit;

Make any rests in the same measure invisible;

Change the barlines before and after the key signature to "Double" and "Start repeat" respectively;

Click on the key signature and reduce the "Trailing space" in the Segment section of the Inspector, until the start repeat barline is in the correct position.