A homebrew system like you're describing is not going to be in compliance, so there's a very strong YMMV with that.
There is no perfect system at this stage of the game--either battery or dynamo. The best you can hope for is to put together a compromise.
My diy lights light up brighter at lower revolutions than my Eyc, so much for "German regulations". At my stage of the game, I don't have to compromise. I can get get battery level brightness with dynamo power without ever having to worry about the disadvantages of battery lights, namely charging them, how they function in the cold, and their limited runtime. I also never carry battery lights for short rides anymore, there's no point, I have all the lights I need.