There are not many options for a step/direction controlled laser. Mach3 or EMC2 are probably the best choices, and as you have found they are better for vector cutting than raster engraving. These programs were designed for controlling CNC routers so that is no surprise. Are you using the laser plugin for Mach3?
The 'dodgy' driver board you replaced was most likely a PLC or microcontroller. The one in the machine may have been a poor example, but in general this is the better method of controlling a laser or CNC.
It sounds like you really want to interface a PIC microcontroller or custom PLC board to your step/direction drivers and write a custom driver to allow the laser to operator more like a printer. This would allow direct output from other programs. Unfortunately that is not a simple task. The slightly easier solution would be to use a driver board from an old printer and interface it's outputs to my motor drivers and laser. The Windows driver would provide the communications as normal.
You could try looking at a Leetro MPC6515 or similar.
Perhaps someone has first hand experience to share on these solutions.