Hypothetically, could JavaScript support be added by using a lightweight engine, such as QuickJS?
[1]: https://groups.google.com/g/dillo/search?q=javascript
However, I don't think the machines that Dillo is targeting would benefit from adding support for it and it would increase a lot the surface for security problems. Maybe my advise would be to use another browser if you have the computing power to run JS.
People tend to overestimate the resources required to run JS and have a miscalibrated sense of how slow they can/should expect it to be.
Firefox 1.x and 2.0 worked on machines with substantially worse specs than many of the ones that people are mentioning here (think: sub-GHz PIIIs with 128–256 MB of RAM), and substantial parts of Firefox itself are written in JS.
I haven't checked, but I'd bet that 2024-era QuickJS is also faster than 2004-era SpiderMonkey at executing the same script.
I was using dillo heavily at the time, when we didn't mind differences in rendering. After that I switched to konqueror. As the dhtml/js craze exploded, I caved in to ff when more often than not the websites didn't load or work as expected.