@erlend_sh I like your solution + @Tom_Newsom’s suggestion.
If I understand correctly, locked topic would still be searchable, but clicking on the result of the search would redirect to the live topic (optional).
In case my English is not precise enough, I refer as live topic to the final thread that merges the duplicates, in opposition to the locked topic)
If I may, it would be nice to redirect to the (first? last?) post of the locked topic in the context of the live topic, instead of the last read post of the live topic.
So that it really gives the impression you arrive on the locked topic, if you see what I mean.