For the bainiku sauce: Remove the meat from "umeboshi: chop it finely and mix in a small amount of mirin and rice vinegar. Pour over the scallop.
Regrading "wasabi" 山葵; I mentioned a bit about this in the previous post. There are two companies which sell "real" wasabi"; one is called "Real Wasabi" another is called "Pacific Farm". You could buy; 1. wasabi daikon 山葵大根 or rhizomes, 2. frozen grated real wasabi in a tube, or 3. wasabi powders made from real wasabi not from western horseradish. Last time we tried rhizomes from Real Wasabi. This time I tried "the grated frozen real wasabi in a tube from Pacific Farm" (left image). I think this is very good and probably more cost effective than buying a wasabi rhizome, although it has some additives like artificial coloring. The other problem is packaging. Initially, the water and wasabi appeared to seperate and then it becomes difficult to squeeze out from the tube but the flavor and heat are very similar to the freshly grated wasabi. It is probably best to smear a small dab on the sashimi itself before dipping in soy sauce but this may be bit too strong for some. In that case, dissolve wasabi in soy sauce. It will keep a long time frozen and, reportedly, for 30 days after opening the tube. In my experience, however, at 3 weeks after opening, the remaining wasabi (about 1/8 of the tube) became almost impossible to squeeze out. By cutting open the tube, I found that the wasabi became bit dry and chalky in texture. The flavor was still there, though. Some of sushi bars in Washington, DC started offering "real" wasabi with an extra charge but I think it is worth it.