An alternate place in the software tool chain to put a 'DXF' button could be in gEDA's gerber viewer program - gerbv. Under gerbv's 'File' menu, there is an 'Export' command, which gives you the choices to save the file as a PNG, PDF, SVG, or a PostScript file. A 'DXF' choice could be added to the list.
This might be a better place than in the gEDA/pcb program, since gerber files are the universal file format for pcb manufacture.
With the conversion happening in gerbv, any pcb CAD program that can create gerber files could use gerbv to turn pcb artwork into .dxf outlines.
If both gEDA/pcb and gEDA/gerbv used the same backend program to internally convert from ".ps to 'path to stroke + union' to dxf", then an 'Export as DXF' button could be available in both programs.