Hi there, so sorry to hear you are having this problem.
I have never heard of such a severe case of fur balls. Most cats are able to bring up fur balls eventually - I've had a couple of enormous ones from my coonies.
Has the vet recommended any kind of malt paste which is supposed to help them bring the fur up?
I have used Katalax before:
Katalax: Amazon.co.uk: Kitchen & Home
I don't really know of any other remedy, other than frequent brushing which you are already doing.
Or is there anything you could do in terms of diet?
I hope more knowledgeable members may be able to suggest something.