Hi!
Well, it might be possible to get the related post objects and index them. For this to work, you will need to use the index table engine:
– Index table introduction
– Index table configuration + generation
– Enabling the index
..and a custom code.
I have put together a small code snippet, but I did not test it yet, so please be careful. Add this to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
Change the following variables as you need them:
– $field_name -> The relationship field name
– $index_title -> true or false, if you want to index the related post title
– $index_content -> true or false, if you want to index the related post content
Once you have the code setup, you need to re-create the index table, and that should hopefully do it.