Hmmm, I’ve always thought of Sakura as being like a really really thick Hollandaise, but it does have a bit of a mayonnaise-y texture to it. (I’ve eaten at the restaurant Drain Bead is referring to.) Haven’t ordered anything with it in ages, though, so I can’t remember the flavor difference.
I used to work with someone who told me that he and a friend had perfected a recipe for it, but I’ve lost touch with him. I’ll see if anyone I know is still in touch with him.