I did a little test in photoshop to see if the above would work well, and the results aren't as smooth as a real box blur, since you see the pixels when you scale the image back up, whereas even a box blur will give you pretty smooth results normally, but for your purpouses, it should be sufficient, since what you're mainly worried about is light sources being missed and left out entirely because they're between pixels at extremely low reoslutions, and not whether the lightmap is super smooth.
I'm pretty sure there is a way to do a real box blur in hardware though. Requires two passes like the above, but instead of drawing one copy of the texture, you draw many like my original blur, but only horizontally or vertically for each pass.
I'm pretty sure there is a way to do a real box blur in hardware though. Requires two passes like the above, but instead of drawing one copy of the texture, you draw many like my original blur, but only horizontally or vertically for each pass.