So does ludens echo work well on kassadin?

it seems it can be good literally on any ap champion

It'd probably be okay, not great. I don't think you get credit for the distance his ult moves him towards gaining stacks on it. So it'd be kind of a slightly better than generic extra little bit of burst damage on him. This seems good, but I'd have trouble justifying building it over the obviously very good boots, archangel's, void staff, rabadon's, or zhonya's. Although it's probably just a tiny bit better than one of those in a few situations. Although if you already have those other 5 and still want more offense, then by all means, go for it.

Nope not really, he can't really stack it fast and since kass has good AP ratios you can just pack straight AP with better utility. If he has enough AP he'll blow up the target anyways, if he doesn't Luden won't help much on him. Sadlife of a champ who has 0 base damages but high ap scalings.