I was trying to find something like this a while back, with no luck. I
even asked Chris at VO if they would consider producing some kind of
stem mount for the silver shifters. He said they had considered it,
but it was way down the list of things they are planning to do.
Best bet would be to find an old stem shift clamp that uses the same
bosses as the current production silvers.
I've even thought about trying to retrofit downtubes to my Sam,
without brazing bosses on. Nothing exists for a 31.8mm downtube,
though, and I don't have the skills/tools to fabricate anything that
would look decent.
-Pete