[eschwartz]

{author_sort:'re(sublist($, 0, 1, '&'), ',', '')'}

Broken down:

        #enter template program mode
{author_sort:'
        #regex
re(
        #Find the first author_sort
    sublist($, 0, 1, "&"),
        #and replace the comma
    ",",
        #with nothing
    ""
)
'}