There was a country singer named Connie Eaton on Welk in and about 1970. Not a regular, but more than one guest appearance. Possibly her.
The “token” country singer on the Welk show was Ava Barber, a blonde who looked and sounded nothing like Ronstadt. Ralna English and Guy Hovis also sang country material sometimes, but again, she doesn’t look/sound much like Ronstadt (and was on the show well after Ronstadt became a solo star).
Maybe people are thinking of Anacani?
I believe there was a very popular poster of her in the 70s wearing a red checkered shirt in this configuration, so you may be thinking of that image.