If the masks drop, some of the passengers will (quite rightly) pull them and put them on, thus you no longer have a fully operational passenger oxygen system.
EU Ops:
(iv) Aeroplanes intended to be operated at pressure altitudes above 25 000 ft or which, if operated at or below 25 000 ft, cannot descend safely within four minutes to 13 000 ft, and for which the individual certificate of airworthiness was first issued on or after 9 November 1998, shall be provided with automatically deployable oxygen equipment immediately available to each occupant, wherever seated. The total number of dispensing units and outlets shall exceed the number of seats by at least 10 %. The extra units are to be evenly distributed throughout the cabin.
If you can get get to your destination at FL250, then I think you could satisfy the regulation, otherwise you'd have to divert.