I just checked the PPC Form 709 Deskbook - it discusses "below-market interest rate" loans on Page 5-3. It says that demand notes cannot be discounted but a term loan can be discounted considering the interest rate for a comparable instrument taking into account late payments or inadequate security. In your case, I guess you're saying that there are no comparable interest rates.
There is a reference to PLR 8229001. Mary Kay Foss