The votes are in (all two of them). With thanks to Emily Selleck and Amy Ivy at the Cornell Cooperative Extension of Essex County over there in Westport, the final determination is that my mystery plant is a species of Filipendula.
Filipendula rubra is commonly known as Queen of the Prairie, a native plant that has a pinkish flower (and it is in the Newcomb's Wildflower Guide - page 234). My plant, however, is not F. rubra, but possibly some other species.
A somewhat exhausting search on the internet has, I think, finally proved successful: Filipendula vulgaris, aka: dropwort. This plant is edible - the young leaves used in salads and soups, the roots, roasted, can be eaten as a food of last resort (starvation food). It also has medicinal properties.
Still, it is not listed in the Newcomb's Wildflower Guide, nor is it losted in Daniel E. Moerman's Native American Ethnobotany. Both of these make me wonder just how long it's been in this country. Hm...