Lots of monitors support an interlaced video input, but they will be deinterlacing/scaling the image to the native rez, not retaining any scanlines. What these people are doing are taking a 240p (sometimes 480i) game output, upscaling it to (at least) 640x480, sending it through the scanline creator, and outputting that to the TV/Monitor, so that it will retain the classic tv/arcade look when used on a higher resolution display.

Maybe I'm mistaken and you really do mean you have a monitor that will generate scanlines on a 240p signal after upscaling it to the lcd's native res - feel free to take pictures / send a product link.