Ah. I seem to have stumbled upon a ludicrously easy way of doing this.
You don't need a grand road made out of dragon soap or platinum blocks.
You just need one tiny rock bridge, and a handful of very valuable mechanisms.
Build a lever out of valuable mechanisms, connect the lever to the bridge using valuable mechanisms, and bingo... road value in spades.
I noticed it when I constructed a bridge as part of an experimental rubbish dumper device. The bridge was connected to the trade depot, but only by stairs, certainly not by a three tile wide road, so it doesn't seem as if the bridge needs to be built anywhere in particular. I connected the bridge to an artifact lever and used masterpiece mechanisms.
I noticed that my road value had shot up, so I built another tiny bridge to test, and again connected it to the artifact lever using even more valuable masterpiece mechanisms (encrusted with various gems, which everyone should have plenty of.)
Building the second bridge caused the road value to jump by nearly 10,000 dwarfbucks. ^^
This seems a bit.. ah... unintended.
I'm quite peeved as I very definitely do not want the King and his Royal Guards - once they turn up I'll be forced to kill them somehow as the "criminal" list encompasses nearly every adult dwarf; noone has been punished for not making a trifle pewter toothpick for thirty years.
(edit - or possibly not have to kill them, it depends whether Royal Guards do Dwarven Justice or not. Fingers crossed!)
(edit again - tried with another small bridge connected to a non-artifact lever. Lever value : 4530 dwarfbucks, mechanism value: 4110 in road, 7890 at lever; total road gain: 16440. These don't add up so not sure what is going on, regardless, it's silly money.)
Would someone else mind trying this out to test it? It may even be bugged somehow as the value gain seems disproportionate to the value invested.