I think you need to distinguish between it being used as a weapon or a shield. If the shield was equipped as a shield (strapped to the arm) you would follow shield bash rules. I would allow a shield to be used as an "improvised weapon" wielded with 2 hands to get the 1 1/2 str bonus but at that point it couldn't be equipped as a shield and if they wanted to use it again I would make the have to utilize the time it takes to re-equip it for the shield bonus.