Getting back to your original posting, I would mention that there is a posting here, where the drill grinder is plastic. Actually, there was one, a lot further back that was just that and crap though it is, my lidl knife sharpener is plastic. The designs might be crap but the principle is sound enough.
Me, well I'm different. I've been messing about with a cheap ,low quality Mig welder and have actually cut some hollow steel section and am welding up a sort of Tinker tool and cutter grinder. the original design was bloody heavy cast iron brackets but cost and one thing and another suggests bolted up and glued mild steel section.
So where does that leave us? First, I think that Mr Hall's design is sort of OK but nothing startling or complete for ordinary tool and cutter grinding. It is no better or worse than one old design in ME that was the forerunner of the fabricated Worden or the cast iron Kennet. I've built both!
Somewhere, I mentioned the Stent which is a baby Clarkson and whilst I built one from castings, I came across a fabricated welded up job in steel sections which, In my opinion makes rings around the quite a few competitors. Again, I like the Brooks which appeared in MEW 16 and 17, years ago and is utilises quite a bit of the arguably complicated Quorn.
Question? Can a Quorn be fabricated? The answer is Yes, an old friend long dead, built one. Can a Quorn be built of plastic parts? I would think so. If you were wary about exotherms, you could do replace the cast iron with a ordinary fibreglass resin- from your local car accessory shop.
Clearly, my knowledge is dated and other alternatives seem possible- but I haven't tried them.
So it would be interesting to hear where you go.