Using Gold as Additional Collateral to lower interest. Does this exist?

[Justin Moon]

All,

I'm always thinking about ways to creatively finance property, and this idea came to my head while I was driving home. I tried using Google to find a comparable example, but I was unable to track one down. 

I am always in the market to purchase more investment properties to rent out, but I want my interest rate to be as low as possible. Is it possible to use gold coins (major ones like Krugerrands or American Eagles) as collateral to buy down the interest rate? Just to clarify, I am not using the gold as part of the down payment. I have a lot of gold bullion that I would like to keep as an investment but since it does not generate any cash flow, I would like to put it to work for me. 

What I am essentially trying to accomplish here is transferring more of the risk over to my side of the fence while keeping the collateral in my portfolio. As the underwriting institution, I feel like this would be a valuable proposition since the upside if quite huge in case I default on the loan. 

Does this type of loan exist? If it does, what is the realistic amount I could expect to shave off of the interest rate and what amount of gold would I have to put down in order to shave off the aforementioned points worth of interest? Just so we have a reference point, let's say the property is valued at $150,000 and I put down 20%. The loan would be a 30 year investment type mortgage for $120,000 at 4.5%.