I’ve looked at the website you gave and it may be possible to do something similar for Shotcut videos.
Unfortunately I got sidetracked doing a fairly large project to produce an app to enable golf club competition secretaries to create and manage golf competitions.
I still have some work to do on the golf app, but it did get me thinking that I could also create an app to simplify creating these Overlay HTML filters. It would be a nice exercise for me to try to do. I’d have to look more closely at the feasibility of doing this, but if it proved too difficult I could always fall back to samples of HTML code that people could edit manually.
I suspect it will be a couple of months before I have the time to make a stab at it,