Last night, shortly after I posted the original plea on four forums in quick succession, I remembered a few more "U" heroines who had slipped my mind at first. Later, as I was about to lie down and get some sleep, "U-Go Girl" popped into my head too (although I was thinking it had two hyphens -- "U-Go-Girl"). But I preferred to keep my ideas to myself for the time being and allow other fans their fair chance to post their own nominations, thus showing how good their memories are for superhero-related trivia!
This evening, I've been going through and collating the results. As it now stands, here are the ones who probably should have been mentioned on that Wikipedia page I mentioned . . . but weren't.
THE SUPERHEROINES
U-Go Girl (Edie Sawyer, a Marvel character)
Ultra Girl (Vicki Grant; this was one of her many temporary "Dial H for Hero" identities in her Pre-COIE appearances at DC.)
Ultragirl (Christine Kelly, daughter of Ultiman in the "Big Bang" universe.)
Ultragirl (Suzy Sherman, a Marvel character.)
Ultra Woman (Lois Lane in a single episode of the old "Lois & Clark" TV series. The episode was very creatively titled: "Ultra Woman.")
Ultra-Woman (An alternate-timeline version of Susan Storm in "What If? #6" (Volume 1). This version of Susan seems to possess essentially the same powers which Reed Richards acquired in the FF's "mainstream" origin story.)
Ultrawoman Beth (Beth O'Brien, one of three people transformed into a superpowered heroic figure in the 1987 animated film "Ultraman: The Adventure Begins," which was intended as a pilot for a TV series, but the series never materialized.)
Umbra (Tasmia Mallor in the Post-Zero Hour Legion of Super-Heroes continuity; before and after that era, she has usually been known as "Shadow Lass.")
Union Maid (Kelly McIntyre, the government-sponsored "national hero" of the USA (the North) in the world of "Captain Confederacy," where the CSA (the South) won the Civil War and remains an independent nation to this day.)
U.S. Angel (Joanie Juniper, the First American's sidekick in the "Tomorrow Stories" title that Alan Moore did for ABC/Wildstorm)
USA, the Spirit of Old Glory (A patriotic heroine who had a short-lived feature in a Golden Age book from Quality)
DOUBTFUL CASES:
Undercover Girl (Starr Flagg, a female secret agent in a bunch of Golden Age stories published by Magazine Enterprises.)
I've never read any of Undercover Girl's stories and I'm far from sure that she really qualifies as a "superheroine," but I decided to at least mention her down here at the bottom. It's possible that I may "upgrade" her status later.
Ursula (A Marvel character, no other name provided, who appeared briefly in "Alpha Flight #108.")
Her big line was: "My name is Ursula, and while I possess no 'super powers' of my own -- my countrymen's latest technology allows me to assume a ceremonial role as I welcome you to . . . the first Pan-European Conference on Superhuman Affairs."
Nowhere is it stated that Ursula has ever functioned as an authentic crime-fighting superheroine in her native Switzerland. That mention of "ceremonial role" makes her sound more like a decorative "booth babe" hired to help out at a convention. (And since that issue, she has never been heard from again!)
Anyway, that's all I've got at the moment, after doing some hasty research to pin down some of the details about suggestions made by my fellow fans. I wouldn't be surprised if we were still missing a few worthy heroines with aliases starting with U, however!