My experience.
I added an Schmidt rear dynamo rear light to my front hub-ed Mercury. The job was classic DIY, for a non-DIY person: nothing really complex, but a lot off fiddly bits. It is relatively easy to do, though tricky to do really tidily.
The Schmidt lamp is small, the seatpost clamp version is very neat. If you get a colour that matches your seatpost it almost completely blends in. SJS have a link on their website to a full set of instructions.
I ran the cable along the hydraulic rear brake line, attaching it with 2.5mm zip ties. A good snips allows a very tidy fastening. At the front you have to bridge from the brake line to a front cable, I used another zip-tie to keep this tidy. Leave a bit of slack so that the cable doesn't get too tight when the steering is pushed to an extreme. The supplied cable is plenty long. You could leave the cable as supplied but that would result in a lot coiled someplace. I removed about 30cm and still had plenty that I wrapped around the headlamp bracket. I practiced cable stripping once or twice on the excess cable. Once the two loose cable ends have connectors in place and are shrink wrapped the "female spade connector" just plugs into the hole in the bottom of the headlamp and the round connector is attached to the bracket.
In use the combination is very nice. The light has good brightness, well out of proportion with size (there are no electric switches or batteries). It can take 5 meters of cycling to light up, but once the bike has been rolling the standing charge means it stays lit for a period if you do stop, something that is useful at lights.
Overall a good experience.
