I'm not a builder, and my only client is an unreasonable fantasist - me. But I'd just drill away and braze on some re-enforcement plates around the hole. After all, no one thinks twice about drilling holes for bottle cage bosses, and a dynamo wire is small by comparison. On my fourth frame I used Di2 grommets for dynamo cable entry and exit.
I'm not a builder, and my only client is an unreasonable fantasist - me. But I'd just drill away and braze on some re-enforcement plates around the hole. After all, no one thinks twice about drilling holes for bottle cage bosses, and a dynamo wire is small by comparison. On my fourth frame I used Di2 grommets for dynamo cable entry and exit.