It was definitely Ned Lagin.
He toured with the dead from around 72ish to 74 and did Seastones with Phil and sat in during some jams. He usually wasn't high up enough in the mix for the audience to hear him though, and so it was mostly the band hearing him in their moniters.... I wish we could hear what he was playing.
That's weird that you saw him, because sometime after the big break from touring, he made the dead sign and agreement that they wouldn't use his likeness or his music in anything.