Proposed Syntax

{
    <path>: {
        $_internalSchemaMatchArrayElement: {
            index: <integer>,
            namePlaceholder: "i",
            expression: <MatchExpression>
        }
    }
}

$_internalSchemaMatchArrayElement matches the array at <path> if the element at index index matches the expression. There is no match if <path> is not an array or if it is an array with insufficient elements.