Can anyone recommend a prop firm that offers overnight leverage of at least 15:1, and what kind of haircut can I expect to pay?
Too many to list. Bright does. Many others do. I suggest you do what I did recently, grab your phone, and start calling: http://www.proptraders.net/523/proprietary-trading-firms/directory-of-proprietary-trading-firms-usa/
You are not disciplined enough to stop trading when you are losing too much money and want software that does it for you, but you think a prop firm should let you gamble overnight? To answer the question, yes firms like Echo and Bright can provide overnight leverage like that, but only if they believe you know what you are doing and have a well diversified and hedged portfolio. A diversified group of equity pairs trading is an example where it can be done, but you will be charged a decent amount on the money you are borrowing.
for 15-1 you will have to be hedged with a select few firms that may offer it echo & bright are your best choices to contact for details
You're going to have to negotiate this based on your book and experience. No firm is going to post a menu offering good commission rates and high overnight leverage that anyone walking in the door can have. Doesn't anyone on ET know how to use a telephone anymore?