If Jane Froman does make a cameo appearance, there appears to be no publicity from the production about it--which is generally why such appearances are staged in first place.
As to post that ID's Froman as woman in red gloves sitting in background behind Thelma Ritter in opening scene, that lady does look vaguely like her--but if it were actually Froman, why bury her in a sea of extras that no one is looking at anyway? Whole point of a cameo is a treat for audience or in-joke--not to fill space in a crowd scene.
Wouldn't be the first time. Maria von Trapp and her granddaughter have cameos in The Sound of Music, but they are so far away Robert Wise has to point them out in his commentary.