NWC doesn't put a courtesy change of Key Signature on the preceding system when the change is at the beginning of the next system.

An almost foolproof way of achieving this automatically is to insert a double bar line (as is required at the change of key signature), then the key signature, and then an invisible single bar line.

If the change occurs in the middle of a system then the invisible bar line has no effect on the appearance, so does no harm, but if the system breaks at the invisible bar line then the double bar and key signature will appear on the system above with an open end to the staff, and then the key signature is repeated as normal on the next system.

The only time this fails is when the system breaks at the double bar itself, but that will happen comparatively rarely. The invisible bar isn't counted and doesn't upset the bar numbering because there are no notes or rests between the two bar lines.