In the same vein as spears charging just like polearms, wouldn't you be able to put as much force behind a polearm to cleave a shield as you would an axe? Many axes are as heavy and awkward as polearms, it's just that shield cleave with a polearm, in addition to strength would require dexterity and finesse.

So maybe allow polearms to shield cleave with a reduced chance of success? (not that it's currently very high ot begin with.)

Just a thought.