Sure, but in a DB plan, a low interest rate or default would not impact benefits, just employer funding.
Re Larry's points, are you saying Larry that you set up loans such that for rate of return purposes (other than the negative rate of a return of a default), the loan is a pooled investment of all accounts, but if there is a default, you take the amount of the default out of the borrowing participant's account? Again, I think you can do that, but I think most plans, maybe nearly all, make the borrowing participant's loan an investment of his/her account for all purposes, so that (1) if equities outperform the loan (which they likely will over a five-year period) the "hit" is on the borrowing participant, (2) you can tell the participant that he or she is paying interest to him/herself, and (3) if there is a default, the loss is borne solely by the participant (which I understand is still the case in the system you describe, if I understand it correctly).
And you don't see a fiduciary issue if (1) the owner takes a loan at prime + 1, (2) equities outperform prime + 1 over 5 years, and (3) because all of the investment characteristics of the loan (other than default) are allocated proportionally over all accounts the owner gets some of the equity return on the money he or she has borrowed, and conversely the non-borrowers get some of the loan interest even though some of their mutual funds were cashed out to fund the loan? It doesn't have to be just the owner or a small plan situation. People feel comfortable based on informal guidance and on the security of the loan and market rates in saying that a relatively low interest rate (e.g., prime + 1 or 2%) is OK for participant loans, but no one would want to invest the rest of their plan assets with a hope of just getting prime + 1 or 2% for five years.