It is important to note that the collateral issue for plan loans is a DOL requirement. ERISA 408(B)(1) requires that a participant loan be adequately secured (among other things) in order to avoid being a prohibited transaction.
Under the DOL regs., a loan is adequately secured if it is collateralized by no more than 50% of the account balance determined immediately after the origination of the loan. As a result, subsequent decreases in account value will not affect the adequacy of the security for the initial loan, but can affect the availability of subsequent loans.
If 50% of the account balance is used for collateral, the remaining 50% is available for hardship withdrawal or any other distribution permitted by the plan. See 17 BNA 1718, in which DOL's Deborah Hobbs is quoted as saying "Under the terms of the regulation, no more than 50% of the participant's vested accrued benefit may be considered as security for the loan, . . . but nothing in the regulation would preclude the remainder of the present value . . . from being available for a hardship distribution."
Note, however, that some plans routinely take a 100% collateral interest in a participant's account balance even though the loan is limited to 50%. In that case, it might be difficult to argue that the "excess" account balance is available for distribution since it is part of the collateral pledged for the loan. Such loan procedures might be modified to reduce the collateral interest to 50%.