Is it possible to run games in Dosbox with a better scaler / filter than the ones available in vanilla dosbox?

DOSBox ECE (Enhanced Community Edition) adds normal4x, normal5x, and normal6x scalers, as well as a surfacepp output mode which does scaling automatically. (These modes all give you the pixelated look of the original game, but at least it's a decent size.)

The software replaces some of the vanilla DOSBox files with the set that enables pixel perfect rendering (which in most games would be 5x). The current version is r4334, but this doesn’t have support for correct scaling. However, they have provided version r4301 (Windows, Linux), which does include this.

To get it working, enable the following settings in the config file:

[sdl]
fullscreen          = true
fullborderless      = true
fullresolution      = desktop
output              = openglpp
glfullvsync         = true

[render]
frameskip = 0
aspect    = true
scaler    = normal5x

Setting scaler to 5x is important; this stretches the pixels to the correct rectangular shape. surfacepp may be used for openglpp, but the latter provides OpenGL support.

I was able to find HQ4x through the magic of the internet archive. Beyond that I see no record of anyone making a higher level of the hq renderer/scaler. Luckily, the downloads still work, and the source code is there even should anyone want to take up the banner and create higher levels of hq.

You could also ask the uploader of those videos for a copy of the hq6x he's using. He may have already put in the work.

Wow, no idea on how he got HQ6x! Searching the web, the most I could find was this 2012 blog post http://ashton-lockey.blogspot.com.br/2012/03/dosbox-linear-scaler-4x-and-greater.html but it only speaks of increasing the normal scaler, not HQ - and I'm not much of a programmer, so I thought best no to try myself.