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?