Putting a few hundred thousand dollars into an investment like bitcoin would be something I'd be happy to do - the risk adjusted returns are within my risk profile. I haven't. The reason is I don't trust any of the exchanges.
Thanks for your input. But I don't think I understand your position. If you would be happy to buy and hodl (as per your statement), rather than HFT, why wold you not: transfer whatever amount you are comfortable to an exchange (e.g. $5000?); buy XBT with the USD; withdraw the XBT to your personal wallet on your computer; rinse and repeat until you've exchanged your hundreds of thousands? You exposure to counterparty risk can be limited to whatever figure you keep on the exchange at any one time.
Alternately, if you really want to take a position of hundreds of thousands, I would imagine you might be interesting to someone like BitPay, whose biz model requires a regular source of $ for which they would provide XBT. You might contact them directly and ask.