Unfortunately not, as far as I know. Lightguns made for CRT screens are not compatible with LCD or Plasma screens.
There are now some emerging lightguns (world firsts, apparently) that are universally compatible with any screen, using sensor 'rails' fitted either side of the screen or a sensor 'camera' that sits on top of it.
While these are an exciting development for fans of lightgun games, I don't know how much luck you'll have finding one that is Dreamcast-ready. The only ones I could find mentioned XBox and Playstation.
It is probably of little consolation that you are not alone; there are in fact many forum-goers out there bemoaning the lack of a flat-screen capable lightgun for the Dreamcast. Perhaps some enterprising chap (or chappette) will get around to adapting a solution for you all. Time will tell.
As it happens, you'll find there are also problems with lightguns on CRTs at high refresh rates - PC monitors or TVs running at 100MHz(+) for instance, are no better than flat screens when it comes to lightguns.
Sorry about that.