I finally had one go, so I tried to weld it back together again. I haven't tried putting it back on, since I'm currently monkeying around with the rear suspension, but it should work if it doesn't break when I put it back on. It may be too stiff now, not able to flex enough to go on without breaking. Or it may work fine. I'll let everyone know how it works out.
Here are the general instructions -
The chrome around the base of the tab has to be cut away. Use an X-acto or utility knife, and go in from the top. You have to get the blade between the chrome and the plastic, then you can twist the blade and peel it off. Be careful not to go past the base of the tab. You don't want the delamination to spread to the rest of the cap.
Clean the dirt and grime off the tab.
Hold the tab in place and tack weld across the crack. Just drag the tip of the iron across the crack and melt the tab back on.
I used the ABS plastic rod, and it worked well. The cap says it is a mix of ABS and something else. ABS alone appears to be close enough.
I put two layers across the back of the tab, which may be one too many. Than I melted through the front of the crack in the tab to make sure I got the new plastic in the back to attach good, at least in that area. I'm a little concerned about the new plastic delaminating, but I think it's fused well around the crack. Then I did the front layer. You can't put too much on there, since it has to snap over a ledge on the rim.
Maybe this repair will work and we'll be able to save these center caps.