GJP I think you may be most correct so far.
The 737 cannot be de-fueled at all stations as the fuel companies do not have the ability to do so. Thus sometimes crews can find themselves in a scenario where an aircraft has fuel on board which was loaded for a previous flight (such a flight being cancelled or flown on a different aircraft for other operational or technical reasons), a flight that can be requiring a lot more to be carried. In such scenario, fly the flight normally, upon arrival at destination enter holding and burn it off, use methods allowed by operator to enhance drag in this scenario. Aim to leave the hold and land at MLW some minutes later. Reason not to fly high drag configuration during flight before entering the hold is that fuel predictions in FMC cannot handle such configurations, thus you may end up burning more than required by doing so.