This proposal has the same effect as adding mandatory empty blocks.

POW targeted at 2 minutes means that the POW for the flag is 25% of the block POW.  That gives a flag every 2 minutes and a block every 8 minutes.

It has the feature that the conversion rate from hashing power to reward is the same for the flags and the blocks.  A flag get 25% of the reward for 25% of the effort.

A soft fork to add this rule would have a disadvantage relative to a competing chain.  It would divert 20% of its hashing power to the flag blocks, which would be ignored by legacy nodes.  The soft fork would need 55% of the hashing power to win the race.

This isn't that big a deal if a 75% activation threshold is used.  It might be worth bumping it up to 80% in that case.

This rule would mean that headers first clients would have to download more information to verify the longest chain.  If they only download the headers, they are missing 20% of the POW.