There will be a serial passthrough in v0.62 though. (only for Win32)
So, of course, it can be done, but I don't know if the DOSBox developpers plan to do it any time soon. You can implement it by yourself, though. The parallel port is emulated internally, and it's output can currently be redirected to the Disney Sound Source emulation, or, with the printer patch applied, to an emulated 24(48)pin printer.