Fearless 

Surely you've heard of stop loss orders & day traders going with day to day mkt. direction .  You have to look at the big picture.  Don't get caught up in day to day swings.  The uptrend is in still in place.

Why do you find it surprising a central bank owning a slug of gold & watching it become  too large a % of their reserves might sell some?  Remember it has come from $35. to $250. to $900.  Isn't that what investing is all about?  I can tell you that if ARU went up to or above $20. I would sell some & diversify a little. That doesn't mean I think ARU is going down.  Would you criticize me for that?  Same holds true for central banks.  Do you feel the ARU insiders are wrong in their current sales?  Same thing with central banks. 

"In which case paying investor to lease gold is surely a good way to go about it. Coinsidently, gold plunged around the same time the rates went negative". 

Can you give me a reference showing a plunge in gold when borrowing went negative?

I suspect it's the borrowing part you don't understand.  If someone is going to lend you gold you have to give them an equivalent amount of money plus a little margin cushion.  Presume you had $850,000. worth of gold when gold was trading at $850. & wished to lend it out.  You would have to get in return cash of about $875,000.  Now you take that cash & buy short T-Bills.  The result is your receiving 2 1/2% on your money until the party returns your gold or you are getting interest on the value of your gold.  Do you want to subtract the negative rate of .2% from your 2 1/2 % & net 2.3%?  Now if the price goes up to  $950.  then you get a cheque for an additional $100,000.  No one is ever exposed to loss in lending securities or gold because the collateral held can be used to repurchase those securities or gold if the other party goes belly up.

"I certainly wish that my broker would give me a negative interest rate charge on my margin account. I might actually use it then." 

In a long margin a/c the broker lends YOU money & you put up your securities as collateral.  You can find plenty of Junk bonds & income trusts that pay you more than the broker charges you interest.  There is your negative interest if that's what your looking for.  Same idea with gold when you lend it out & pay a very small fee in order to use the cash to get a higher rate of return.  Apply yourself.  I hope you will have a better grasp of borrowing & lending.

Regards

Joltin