One trick that *may* work is this:
Depending on the nature of the dent, there is likely to be a crease, leading to a corner on one or more sides of it. When the dent is caused in the first place, it usually starts by pressure being placed by an object(eg, child's hand), and moving across it. This corner is the the last part of the dent that occurred. What you want to do is reverse the process.
To do so, you want to very gently apply pressure on the cap on the undented part - near to and toward this crease. Do this such that you're not making another dent, but rather you're deflecting the cap just enough that it forces the dent out from the back - a little at a time. If you do have a "corner crease", you want to work on either side of it, because it will be hard to overcome the tension directly in that corner.
As noted above, you don't want to apply too much force that it will mis-align the voice-coil structure. If you're lucky you can work it most, or all of the way out. I've done this on a dome tweeter, and on car fenders (which is where I learned it - working my way through college as a body repairman).
I hope this makes sense - it's easier to show than to describe. It won't work in all cases, but does in many.
