I think the bitcoin economy is really too volatile for giving out loans. Only about 50BTC are generated every 10 minutes. With rapid deflation you would need negative interest. The problem is that it is more profitable to buy and hold, rather than giving a negative interest loan.
For planning, I am using the 30 day weighted average price. I am assuming that should even out most of the volatility.
You can pay interest without problem in a price deflationary economy. You just need to invest the money in a business that generates income (which is the point of a loan). That is the problem on why nobody is lending in bitcoins right now, because its hard to generate a business that generates income in bitcoins.