I suppose the technical answer is the loan value as of the earlier of 1) the earliest date a distribution could be made, or 2) the date of default. If distributions are available on the date of termination of employment, I would definitely not accrue through the grace period. If distributions can't be made 'til...the end of the year, then you're looking at the default date, which is possibly the date of termination, if the loan policy says loans must be repaid immediately upon termination of employment, or the end of the grace period.
But, the practical answer is - most recordkeepers aren't going to credit (charge?) interest unless a payment is made, so it's going to be whatever the value was as of the last payment. And that is close enough for government work in my book.