Unofficial instructions for servicing Shimano dynohubs can be found if you google around. It's far from rocket science but relative to other bike mechanicing jobs it's what I would describe as a difficult procedure. There are more bits than a regular hub to put back together again, and more things you could break (the wire, the stepped threading). And like you said, it's tricky to get preload right because the magnets make it hard to feel. That said, with lots of kms piled up on your dynohub it's nice to know that you have new bb's, clean grease and no pitting in there. When I opened mine at 15 000 kms the grease had gotten pretty gross looking.
It's almost impossible to find cones
afaik. The dynohub axle is extra fat to make space for that wire and so they made special cones to fit on it. If you do manage to source some please let us know!