You can do iiiedokeiii's install script, which prevents matchbox from restarting; this isn't necessarily the best option though, because if matchbox dies from a crash or something, you could be stuck with no window manager at all.

If you read through the whole thread, you'll see a couple of options to avoid that; the one in post 15 is what I currently use, as I think it's the best option.

I think the CPU applet iiiedokeiii mentioned is the same one I use; it's full name is osso-statusbar-cpu. You can also use Personal Menu or Personal Launcher to run arbitrary commands.

With any of those, you should be able to select a menu item that will run a script; you can have that script either shut matchbox down cleanly (as above) or kill it, as you like, and then start FVWM. You can also have another entry to stop FVWM and restart Matchbox.