The reason this can be done with Dreamcast VGA boxes when used with a Dreamcast is that the box doesn't actually do any conversion itself. The "hack" involves telling the Dreamcast to generate signals at either CGA or VGA timings. It's apparently pin 6 on the connector (according to the docs I have).
Also, just because a device can downconvert, doesn't mean a device works the other way, though with a simple deinterlace (line doubler)/interlacer, the hardware is similar.