Make custom textfield autocomplete compatible with slashes?
Take a cue from the Drupal core's taxonomy autocomplete implementation and just recombine the arguments passed into callback function back into one string:

function taxonomy_autocomplete($field_name = '', $tags_typed = '') {

  // If the request has a '/' in the search text, then the menu system will have
  // split it into multiple arguments, recover the intended $tags_typed.
  $args = func_get_args();
  // Shift off the $field_name argument.
  array_shift($args);
  $tags_typed = implode('/', $args);