Would this be useful for a bat that is ridiculously hard pressed? Ie would this eventually "open the bat up"?
I dunno, feels wrong to knock in a bat purely to 'open it up'. I think netting with it is more effective, as this will loosen the glue in the handle, which will make the bat vibrate less, and essentially make the bat feel better on a whole. If a bat is a dud, no amount of knocking is really going to make it any better. You may as well get as much value for money as you can out of a firmer pressed bat and use it in the nets when you've had it prepared properly. One thing I've found is that if you face a new ball with a GM, it makes a cracking sound!
I think that there was an article which was talking about how older bats which were slightly firmer pressed took a while to delaminate, but was worth the playing in as it showed that the bat was at its very peak. (I think it may have been Ramprakash talking on bats) Many professionals also like it when the handle is just about to go, which is when the bat feels like it performs the best.