Since YouTube was considering my DOSBox 640x400 captures as only deserving 360p, I set myself to do something about that. 360p is just not enough for showing tracker commands; everything comes out too blurry. The answer, as usual, was ffmpeg.
This is what you need to upscale a DOSBox video from 640x400 to 1280:800 (double its original size):
ffmpeg -i dosbox.avi -vcodec libx264 -vf "scale=1280:800" -r 60 -vpre lossless_max -acodec copy -threads 3 resized_output.avi
The -threads 3 option is totally optional. It tells ffmpeg to split the encoding work into three threads, which is useful if your computer has three or more cores.
Here's a sample of a song captured and upscaled this way, running Impulse Tracker 2 inside DOSBox:
(For best enjoyment play it full screen!)