For exports, or delivery to domestic customers too?
For exports it could make sense, assuming they have the sense to limit their commitments to delivery of small volumes no more than a year or two from now. In a few years, their production available for export will be negligible, so if prices rise, they would not lose much by sticking to the contract. But bearing in mind their collapsing production, they would have to be very careful about promising to deliver volumes they may find themselves unable to deliver. That way lies a
very messy default. That said, if oil prices fall as they expect, that final trickle of exported oil could prove to be more lucrative than would otherwise be the case.
If this involves the domestic market at all...
It would certainly be an innovative tax, locking domestic consumers into paying today's high prices in tomorrow's low price world. The result would be foreign competition, and if unsuccessful, revolution.
But if tomorrow is a higher price world, it would lock PEMEX into a straight subsidy of domestic demand. This would force the rapid disintegration of the company's assets through neglect.
...there is no way to win.