Legal requirement for measurement equipment calibration in manufacturing
IANAL, but it seems to me that the only legal impact of using an out-of-calibration power supply would be the extent to which it violates the terms of the specific contracts you have in place with your customers.

Outside of that, you simply need to evaluate the risk that the power supply will actually be outside of its performance specifications, and the extent to which this will affect the specifications of the gear you're producing and/or testing with it.

It isn't as though the power supply will suddenly change its characteristics on the day that its certificate expires &mdash; the date on the certificate is just a conservative estimate on how fast the unit's parameters drift or age with time. You could conceivably go ahead and do the production run past the date on the certificate, and then have the calibration lab tell you whether it was in fact out of calibration afterward. If not, then you have no exposure at all.