New paper by Lucas (not that one) and Serrano. From the abstract:

This paper explains the “positive profits with negative surplus-value” example of Steedman (1975) and shows that while in joint production systems individual labour values can be negative, the claim that the total labour embodied in the surplus product of the economy (surplus-value) can also be negative is based on assumptions that have no economic meaning (such as negative activity levels). The paper also provides a way to measure the surplus-value of joint production systems which overcomes the problems of the traditional concept and restates the proposition that a positive amount of surplus labour is a necessary condition for positive profits.
Read full paper here. A preliminary version was briefly noted here in 2012. Academic publications are slow indeed.