One company that does that is http://epiphyte.us (which I happened to speak with today, but I'm not affiliated with them):
We provide customers who are looking to enter the digital asset space with
deep, off-market pools of liquidity. Through these liquidity pools,
we operate as broker/dealers for large blocks of digital assets.
Edit: Also, a thread on Reddit about what appears to be a big off-exchange sale of 117,000 BTC that was supposedly related to Fortress's Bitcoin fund:http://www.reddit.com/r/Bitcoin/comments/1u4fc6/record_numbe...
Alternatively you can just place a market order a few times per day for a week. It's more work, but we're really only talking about ~1 minute of work per $200k or so. MtGox handles more than $10M in Bitcoin sales per day and they represent a minority of all exchange volume.
Other exchanges would also have similar issues, and would hit you with all sorts of antimoneylaundering questions if you try to withdraw too much
https://bitcointalk.org/index.php?topic=356026.msg3835542#ms...
As KYC questionnaire a company has to answer here is alot longer than above, I am not sure if allowed to post it, but it is very thorough
All those new cashing out corp/merchant services like Bitpay and Coinlab don't have any limits either to the amount you can turn into fiat