does anyone have any good tips or drills to flatten out a players swing path?
I have a student who has come to me and has a full western forehand grip and has a very vertical swing path, even on short balls. He is struggling to keep the height of the ball down while still hitting it aggressively.
It seems like you're talking about two issues (please correct me if I'm wrong).
If he just needs to bring the ball down he can close the racquet face a bit. Given the same vertical swing path he'll get a ball with a lot of topspin, but lower. This is a super useful shot at times, especially on lower short balls where you have to have the spin to keep the ball in if you're going to hit it hard.
OTOH, if he's looking to hit more through the ball, i.e. a ball with less topspin and more energy going into pace, that can certainly be done with a W grip. This is not a shot that I'd use on a low short ball because you'll tend to hit it long. But it's great for more power from deeper in the court, or on a ball above the net that you can pound. Basically (regardless of grip), the racquet needs a greater component of its velocity going into the ball as opposed to brushing the ball. With that grip he'll have to allow his wrist to have more ulnar deviation to increase that part of the motion of the racquet. Note, you're not forcing the wrist to move that way, you are allowing it freely swing in that motion. The power still comes from the core and legs. It's a little tricky to do.
I'm not sure how to coach this. I don't think I'd get into the complexities of wrist motion. At the end of the day you're still just creating a swing path that moves the racquet head more into the ball as opposed to brushing up the back, and for the player that's probably the best way to think of it.
It's a bit easier to get that into the ball motion with a SW grip (as you said) because the that same swing path now involves more flexing of the wrist. But with all of these motions you're still mostly just allowing the wrist to move, not forcefully contracting the muscles and making it move in that way.