When the water is high, Curtain Falls most definitely can produce rainbows.  It is in fact named for the curtain of mist that it produces during high water.

If I may be permitted an opinion, it is a most fitting spot for scattering ashes of someone who loves either the BWCA or the Quetico, being on the border of each.  It's also a very historically significant place for both the native Americans and the early explorers and fur traders of the Europeans and Americans.

I would recommend a May or early June trip to maximize the chances of seeing the falls in it's full splendor.  It's also quite easy to get to by taking a tow from Crane Lake to Bottle Portage between Lac La Croix and Iron Lake.  Could even be done in a single day if need be.  Catch the first tow out in the morning and the last tow in the evening, though doing it in only one day would be robbing the participants of a wonderful experience, IMHO.