gistfile1.rb

module Capistrano
  class SSH
    class << self
      alias :original_connect :connect
      def connect(server, options={}, &block)
        special_server_settings = options[:server_authentication] ? options[:server_authentication][server.to_s] : nil
        if special_server_settings.nil?
          return original_connect(server, options, &block)
        else
          methods = [ %w(publickey hostbased), %w(password keyboard-interactive) ]
          password_value = nil
          ssh_options = (options[:ssh_options] || {}).dup
          ssh_options[:username] =  special_server_settings[:user] || server.user || options[:user] || ssh_options[:username]
          ssh_options[:port]     = server.port || options[:port] || ssh_options[:port] || DEFAULT_PORT
          begin
            connection_options = ssh_options.merge(
              :password => password_value,
              :auth_methods => ssh_options[:auth_methods] || methods.shift
            )
            connection = Net::SSH.start(server.host,connection_options, &block)
            Server.apply_to(connection, server)
          rescue Net::SSH::AuthenticationFailed
            raise if methods.empty? || ssh_options[:auth_methods]
            password_value = special_server_settings[:password]
            password_value = password_value.call if password_value.is_a? Proc
            retry
          end
        end
      end
    end
  end
end

gistfile2.rb

role :app,          'thisapp.rocks.com'
role :web,          'thisapp.rocks.com'
role :db,           'thisdb.rocks.com'
role :assets, 'my.asset.server01.com'
set :user, 'the-primary-user'
set :server_authentication, { 'my.asset.server01.com' => {
                                :user => 'assetusr' ,
                                :password => 'password'}}
                            }