Am I right in thinking that this form of relative cannot be used at all after a preposition?
Yes, you can use these relative pronouns after prepositions. We have two lessons for this; they are more focused on "el que, la que, quienes.." but it would also apply to "el cual, la cual...", for example:
La empresa para la cual trabajo me ha propuesto un proyecto muy interesante.
The company I work for has proposed a very interesting project to me.
Have a look at the two lessons:
Relative pronouns with prepositions
Relative pronouns with prepositions a + de
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