Can wp_insert_post_data filter be used to save custom field data?

You can access the raw $_POST data from the wp_insert_post_data filter, but there are probably better ways to intercept that data.

save_post or similar hook would probably work
As you are dealing with post_meta, update_post_meta uses
update_metadata which provides the update_{$meta_type}_meta
and updated_{$meta_type}_meta filters. Those may be the best for
you but your question is shy on detail so I can’t be sure.

Based on the edit, use the update_{$meta_type}_meta filter. Your question is still light on detail but the following filter should dump all of the data so you can at least see what you are dealing with:

add_filter(
  'update_post_meta',
  function($meta_type,$object_id,$meta_key,$meta_value,$prev_value) {
    var_dump($meta_type,$object_id,$meta_key,$meta_value,$prev_value);
    die;
  },
  10,5
);

Note: that will break your site but not permanently. It is purely for debugging/development. It is just to give you a glance at the data that comes through the filter.

Related Posts: