Relationships is quite another beast to handle imho. I know e.g. ola has developed a relationship plugin, but as I expressed my concern, it doesn’t handle “keep established relationships working” well (e.g. in case of url renames or page deletions)? You’re on your own for accounting this, via hooks and other plugins 
. This makes the custom development dependency graph quite huge for a little use-case; imho it feels like building a house of cards.
Thanks for your insights in this, @texnixe.
When I find some time, I might have a go at your other proposal about building a “crawler” ourselves and searching its built cache then. Performance will be a pickle I’m afraid. Maybe cron can help with that? Does anyone know of good, simple, open source PHP crawler that we could plug in for this? I would like to avoid re-inventing the wheel.