I'm new here, and don't claim to be a bitcoin expert... but for what it's worth, I do think this idea has potential. Loans are what drive economies, and without the ability to get a loan in BTC, it would be hard to have a large number of organic, bitcoin through-and-through firms.

But yes, it is hard to manage a lending institution in a highly volatile currency. The fisher equation simply states that banks should set their interest rates at
nominal interest rate=desired interest rate + expected inflation
In bitcoins, the inflation (or, in this case, deflation), is ridiculously hard to predict, so this lending is basically a highly risky betting game.

The advantage of bitcoin is that people expect unexpected things to happen to the exchange rate, so I would argue that a lot of people would be OK if all interest rates were variable.

A bank chooses an indication of inflation (for now, exchange rate relative to some currency might work), and sets a real interest rate. All loans are variable interest rate, found by the fisher equation.