Has anyone EVER successfully repaired shockmounts on their own?  

Cwoo
You can contact Graham Mancha in the UK or Alfie at HumeModern in the US. They are the unquestioned best people who are repairing Eames today. Graham is a little bit more customer service oriented than Alfie is. He will sell you glue, a new shockmount, and tell you exactly how to do it.
The metal disk on the inside of your shockmount is what the bolt threads through in order to hold the backrest/seat on.

use PC7
That will do it...
http://www.right-tool.com/pc7epoxypaste.html

You can get new ones cheap
Why bother repairing the old ones when you can get new ones for 12 pounds?
You can also order shock mounts from any Herman Miller dealer, too.
http://www.mancha.demon.co.uk/spares.html

Its not so much the...
Its not so much the shockmount itself, the question I had in mind was more on how to affix it. Will look into the epoxy.

I would get
I would get the glue and ask some advice from Graham Mancha at the links above. I've seen probably 20 or so various plywood chairs with repairs done to them and NONE had a nice clean look. Almost always the epoxy was gooped on around the edges like frosting on a cake.

If you receive any...
If you receive any additional information, please post it on the forum. This issue has been a topic on the forum several times.

i agree with what lucifer...
i agree with what lucifersum says we have had a lot of chairs shipped in and they had the frosting drip in black and it is awful it is also almost impossible to clean up
try the p7 epoxy it should work
we have been using the clear silcone and it has been working so far, I think you can buy p7 at Ace hardware,

glue?
Its not so much the shockmount itself, the question I had in mind was more on how to affix it. Will look into the epoxy.
= = = = = = = = = = = = = =
When I switch the standard 4 legged base on a nice upholstered Eames chair with a nicer LaFonda base (!!), I noticed that two of the shock mounts were unglued, I just screwed the LaFonda frame to the shell without gluing the mounts and they're snug and working fine.
Is it essential to preglue them?

PC7 works
Just to throw my 2 cents worth. Yes, PC7 works a charm. I had a shockmout that kept coming off. I tried a few types of superglues and epoxies with limited success. PC7 works as it will bond to both fiberglass and rubber.

Yup use PC7
I reglued mine about 7 years ago and it lasted that long as my daily desk chair, and finally loosened up a bit ago. Slapped some more on and it's good to go again!
edit: my chair is a DCW also, not fiberglass...PC7 will stick to both wood and rubber (and fiberglass). Make sure you mix it well and let it dry for a day or two.

PC7 did the trick for me...
PC7 did the trick for me too. Too early to tell whether it will stand the test of time, though. Thanks all for your tips!