Hi Hannah,

en lequel is grammatically correct and is indeed used. Here is an example:

Mon ami, en lequel j'ai confiance, habite à Paris. -- My friend, in whom I trust, lives in Paris.

Does this help? -- Chris (not a native speaker).

This makes sense, thank you. 

Can you also use “en lequel” with objects?

And is there any specific reason why you would use “en lequel” rather than a different preposition?

Thank you for your help :)

Yes, you can use en lequel with objects. Actually, it is mostly used for objects because for persons there exists an alternative construction.

The woman I work for. -- Le femme pour qui je travaille.
The company I work for. -- L'entrepreise pour laquelle je travaille.

You might want to check out this lesson for further details:
Using [preposition] + qui/lequel/laquelle/etc = on what/behind whom/beside which/etc (French Relative Pronouns)

You're also asking why I would use "en" and not a different preposition? Which preposition you use depends on what you want to say. In principle, any preposition can be combined with laquelle/lequel/lesquels/lesquelles.

L'arbre sous lequel j'étais assis. -- The tree I was sitting under.
Les livres sans lesquels je ne peux pas continuer. -- The books without which I cannot continue.

I hope that helps, -- Chris (not a native speaker).

So why isn't en lesquelles ok here, please?

So why isn't en lesquelles ok here, please?

Not ok where?

The en is used when "de" is necessary in some fashion whether needed with a verb that takes "de" or when replacing a partitive.

The en is used when "de" is necessary in some fashion whether needed with a verb that takes "de" or when replacing a partitive.