you can use a razor to cut off the dustcap, and then use "Goop" to glue it back on.
once you cut it off though, i would just buy a new one from SI. considering how stiff the dust cap is, even if you get the dent out, the crease will stay there.
i did the razor thing to my btl. works fine. you just need to go around the glued perimeter of the where the cap meets the cone. either score the CA glue all the way around real good and pop the dust cap off with a screw driver (ie actually hit the dust cap and it should break the seal), or instead of scoring it, just go all the way through it.