Wheel building - ever encountered nipples that are screwdriver-only?
I'm not sure about building a wheel using just the screw slot – I think you could probably do it, but I suspect that it would be a painful process. If you tried it, you'd want to be sure that the spoke lengths were exactly right. Much of the strength of the spoke comes from engaging with the threads in the wider section of the nipple that is on the inside of the rim.

Hi-E used to make nipples that were driven from the inside with a hex head. I built a number of wheels with them, they were easy to work with – the only downside is that you have to remove the tire to true the wheel. But for you, that would be a feature, you don't want somebody messing up your beautiful wheel with a spoke wrench. I think that would be the way to go. Hi-E is gone, but you can still get them from Sapim. They make alloy nipples in a number of colors including black and orange.

You could also consider using "inverted" nipples which are installed on the inside of the rim (only the spoke shows) if you can't get the right orange.