I really don't see what the cost of a .22 Hollowpoint has to do with trying to improve it's effectiveness. I mean, just because they are cheap doesn't mean we can't try to make them better. But being cheap, experimentation should be the rule of the day.
I also don't see how enlarging the hollow point cavity would make the bullet keyhole.
But, try it, as was mentioned, they are cheap, give it a whirl and get back to us with the results. Cut a few, shoot them on paper, see if it keyholes. Shoot a jackrabbit or something and recover the slug, see if it opened up more than a regular HP. They are cheep, experiment.
When I was doing this, I would have considered it a bonus if they keyholed. We were shooting at stuff that was maybe 10 yards away, but probably closer to five. Hitting a small animal with the side of the bullet would have certainly improved it's effectivness. We got so used to sneaking up on stuff with pellet guns that we continued to do the same thing for years with much more powerful weapons from .22s to rifles, to shotguns. I distinctly remember saying, hey George, you know, we don't have to sneak up to stuff like this anymore, we can shoot from here. And it was kind of a revelation to us both. I remember turning down shots at deer because they were like 50 yards away. I was so used to getting as close as possible, that I considered that to be a sniper shot.