I found this whole discussion a little humorous. For us old folks who worked on plans in the days before daily valuation and employees being responsible for self-directing their accounts, loans were ALWAYS just another investment in the plan and the interest from the loan repayments was part of the overall investment return that was allocated pro-rata to all participants. When I started in this business, this was everyday plan accounting.
Of course, it is not as common now with daily valuation and self-direction, but it is absolutely permissible. In a pooled account, the loan is a plan asset, like any other investment, but if the participant defaults, then the defaulted loan amount is only assigned to the borrowing participant, meaning the participant's account is offset and the participant gets a 1099.