Quick update on my original post:
It looks like I just didn't fully understand how radial gradients are intended to work in Scribus. After some further experimenting, I was able to get what I want with the following steps:
Select "Radial" as the gradient type
Use only TWO stops (not three as I did originally)
Set the left stop color, opacity, etc. (100% white in my case)
Set the right stop color, opacity, etc. (100% black in my case)
Note that in the Properties dialog window, you will see what appears to be a linear gradient, white to black in my case.
Basically you are defining the gradient pixel values transition from the CENTER of the gradient to the outer edge, and that's what the graphic in the Properties dialog is visualizing, NOT the final appearance of the gradient in your text frame.
Now the really important part: Use the Move Vector dialog to position the start, end, and focal points for the gradient. Typically, the Start and Focal settings would be the same if you want a perfectly centered radial gradient. The Start, End, and Focal parameters are X/Y pairs, and the values are relative to the text frame (it would really help if this were more clear in the dialog).
You have to visually watch the text frame, NOT the gradient in the Properties dialog to understand how the Move Vector works.
Whew, this really needs to be a bit more obvious in the UI (IMHO).