Is it possible to use scalers like 2xsai or hq2x in GOG dosboxed games i.e. EWJ 1&2?
I think it will greatly improve player's experience.

It should be possible. I don't have the game installed on my laptop, but it uses dosbox, so it should be a simple matter of finding the dosbox*.conf file in the game directory and opening it with notepad or something similar.
Find the line called "scaler=normal2x"
You can change it to something like "scaler=hq2x" or hq3x or 2xsai or super2xsai or more.
It should be in the conf file what options you have.

For 3D games it works very poorly, but for games like Ewj it should work well. Just remember the more different colors in a small area, the more confused it might look. I've tried using hq2x for X-com, and it ends up looking like someone tried to do an oil painting. Not really bad, but it might be distracting for an action game like ewj. Best way to find out.... try it.

Try EWJ1.EXE 320x240 as well

Why don't people like giant pixels.

Forget all these scalers and play it in all its pixelated glory