What happens if I use the ability of Mind Raker and the like to put my opponent's cards from exile back into their graveyard with Leyline of the Void in play? I assume it keeps the card exiled permanently and I still get the benefit? If so, that would be a pretty useful combo.

yes.

I thought so, thank you.

Be aware that using this combo will make the exiled cards become 'newly exiled' (rule 400.8), so if it had first been exiled by cards like Stasis Snare, then Stasis Snare loses track of it and can never bring it back (rule 400.7).