Thanks for the various ideas Claymore (Brian). I like the resins idea, though I THINK I remember seeing something similar on a US site (could it have been Mike's Workshop? or Steve Goode?).
Not tried resin myself, though I MAY get round to it "one of these fine wet days"
Another idea I've already tried which worked OK (provided a VERY fine-toothed metal cutting blade is used - Pegas have them) is cut sheet metal.
I've cut brass, ali, tinplate, MS, and nickel silver (called "German Silver" sometimes), all in varying thickness from about 10 up to about 25 thou (inches) so far.
You need a sacrificial under-sheet (thin flat ply is best but dense cardboard works too), taped up with clear parcel tape, and quite a slow stroke speed + v gentle feed. (And it's also quite noisy)! But it does work well, and is very accurate if you go slowly enough.
Note I have not done this for "normal" scroll saw work but rather for knocking out components for other uses (such as oddly-shaped model aircraft fuel tanks, model loco and wagon sides for someone, etc). But I think that "nicely done", a polished nickel silver insert into, say, a key ring or other name tag type thingy could look quite good, though I haven't really thought about exactly how to do it.
BTW, the above sheet materials are quite easy to come by in various thicknesses at model railway shops, (sheet plastics too, though I haven't done any of that) and although the sheet metals are quite expensive (especially nickel silver) I don't think you'd need much for the above ideas. And also BTW, nickel silver is lovely looking stuff - a sort of silvery light brassy colour, and surface corrosion/discolouration is not a big problem - it stays "half shiny/half matt silky" for a long time, and I guess a fine coat of very quick-drying clear varnish would reduce that small risk to nil.
I STILL have not started my aeroplane Intarsia adventure yet (!!!!!!!) but have already been thinking of some small sheet metal parts for aircraft areas which might need it. I'm not sure how it would look as a final result though, nor have I given much thought to exactly how to incorporate it either, but in the same vein as your post (which was certainly NOT too long) I thought I'd share the above.
The big disadvantage I see for anyone who wants to get "commercial" is that I've found cutting sheet metal VERY slow, so I doubt anyone would come even close to covering their man hours.
One final thought I had (which HAS been done before, but I haven't tried it yet) is wood burning to add texture/s & small details - again my aircraft intarsia springs to my mind.
Thanks for a thought provoking post, I hope the above may also be of some interest.
AES