Most of the autograph groups on Facebook are bunk but one or two people reported success with the Vienna Concert. I replicated it (thankfully I worked in shipping and receiving for 20 years), and got results.
Basically, I sent it like you said - but I made sure to give it some extra time, so I think it got there 2 weeks ahead of the concert? I also sent it Second Day FedEx (Human nature: FedEx and UPS boxes inherently look more "important" than regular USPS envelopes) and inside was:
* A prepaid FedEx Overnight Envelope, with some added cardboard to keep everything flat.
* A short letter to whoever opened the box at the venue, politely asking them to pass along the rest of the contents to Mr Williams if and only if it was unobtrusive. If not, to please ship them back to me in the prepaid envelope.
* A short letter to Mr Williams
* The CD cover
And months later, having given up on it, I got a random FedEx notification that the return label was being processed through a hub.