Re:
Jim, this was years ago so memory of what I did might be faulty, I had a spinning rivnut in an old Cannondale hardtail frame.
No bottle cage just the wee bolt which was the top one in the seat-tube. I got a long thin screwdriver, I think, something with a thin narrow edge like a screwdriver anyway and pushed it down hard against the part of rivnut inside the seat-tube to jam it in the hole in tube then carefully unscrewed the bolt. Had skooshed WD40 on threads of bolt inside seat-tube and left it to soak for a while. If this hadn't worked I had thought of cutting a longish V notch with filed "grippy" bits, like a pair of pliers, in end of thin strip of steel to jam around rivnut inside frame to hold it.
You probably know this trick already, I then tightened the rivnut up using a longer wee bolt than bottle cage bolt with a nut and washer on bolt. Screwed bolt into rivnut till threads fully engaged, washer is fitted between nut on bolt and rivnut, hold bolt then tighten nut against washer/rivnut to crimp rivnut tight in frame.