The Panasonic saddle and original frame clip I ordered online, as well as a Shimano standard brake lever for mamachari (cheap mini-cycles) arrived. 
This is what the Panasonic tail-up saddle light (I ended up choosing this product, part number NSSS046) looks like with the seat post metal fittings removed and from the side.
It might need to be remove the plastic cover on the back of the tip. 
My ideas and fantasies (and online shopping) are progressing well, but the rainy season has started in Japan, so unfortunately I won’t be able to maintain or ride my bike for a while. 