I do not know if there are articles about the organic acids found in black water rivers.
Many people keep (and breed) black water fish using peat moss or driftwood as the source of tannins and other organic acids. Tannins will color the water anything from yellow to brown. You might look up some of the breeders of Discus for more info: Hans Discus, Jack Watley, and web sites that specialize in Discus to see what (if anything) they do about trace minerals or salts in the water.
Check with the person who is breeding the Discus for the optimum water conditions. Several generations removed from the wild will probably mean softer water, though not quite so picky as wild caught. Until you hear from them I would aim for GH and KH of about 3 degrees, pH on the acidic side of neutral, perhaps the mid 6s. TDS about 100.
You might try some test batches of tap water + RO to see what it takes to get into the right range on the water parameters. If you have the soil on hand, add a handful of that, too. See what happens. Most testing materials at the hobbyist level do not include anything for the trace minerals, but you can probably get kits for:
N, P, Fe, GH, KH, pH, and get a meter for TDS. (I have seen them on line for about $20.00 US, I do not know what the price might be in the UK)
Barr's GH Booster is available through:
It is very similar to Seachem Equilibrium, but lacks some of the trace minerals that the Seachem product has.
I think your idea of adding the traces to the soil is good. Yes, a little bit will leach out into the water. If you are planning on doing frequent, large water changes you will want to fertilize the soil, anyway, and not bother with adding ferts for the plants to the water.
As for the 'network of buried, perforated tubes' I would skip it. When you want to add fertilizer tablets just push them into the substrate by hand, or with an injector (sort of like a syringe).