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I'm aware that grapes are rich in DAP and therefore making wine requires no nutrient addition before fermentation. Since sugar washes have no nutrients, I was wondering whether adding say, 5% pure grape juice would provide sufficient nutrients for a sugar wash? By 5% I mean, if I were making a total of 100 L wash, it would contain 5 L of grape juice and 95 L of sugar wash. Is the DAP present in the juice mainly or in the grape skins? Along with some acid (like citric) to reduce pH to around 3.5-4, would this result in a nice, clean fermentation?

IMHO 5L of juice for 95L of wash will not support yeast in nutrition suitable to ferment all sugar (aming for 10-12%ABV). Another thing 3.5-4 PH is to low try to keep it in 5-5.5 region. Visit local brewer shop to buy DAP (or get it online) use it as it says on a packing also you may wanna check Tried and True section here on HD lot of good recipes on sugar washes there, like WPOSW or Gerber. My experience shows that small amount of nutri + low PH = stalled ferment. It's always easier to get good clean ferment from the start then to restart stalled one.

Hmm, juice from most grapes are between 3.5 - 4 so I assumed that that would be a healthy environment for yeast. But that's a minor point. If grape juice DOES have any significant amount of DAP in it, I'd actually be perfectly happy to even use 15% to 25% juice (85% to 75% sugar wash). Definitely looking to get a nice clean ferment from the start, and shooting for 10-12% ABV, as you suggested. I'll check out the Tried and True section.

Try it on smaller batch then work your way to 100L. Juice has lot of other stuff besides dap so yeast can ferment better.