Take, for sake of concreteness, the preposition entlang (see also this related question).
At least, two uses are possible:
Genitive preposition: Entlang des Flusses gibt es …
Accusative postposition: Den Fluss entlang gibt es …
(I ignored the dative-variant since it is rare and it's something don't feel confortable with. But the answer explains it all.)
As for a relative clause, is genitive the only option?
der Fluss, entlang dessen es … gibt, …
or is it also possible with the accusative variant? like:
der Fluss, den entlang es … gibt, … (which somehow sounds awkward).
I guess the same rule would hold for other postpositions, like wegen.