The Central bank can police the internal workings of its regulated banks. It can look at the repo'ed portfolio a bit at a time, by sampling. Remember the commercial banks receive liquidity from short-term (weekly, in the ECB's case) repos. An asset that was accepted last week can become unacceptable next week, or experience a higher haircut.

It's a bit like quality control or customs inspections. You don't want to inspect every item in every box in every container but if you sample and do find a faulty item you can quarantine an entire shipment pending further investigation.

Evidently, running the discount window properly would require a lot more manpower than banking supervisors currently deploy...

Economics is politics by other means