My 2 pfennig:
Look at the charts (table?) for the highest TOGW under the existing conditions (assuming full power, no derates/flex which would change everything) and use that V1 as the V1max. It is a rough approxiamation, but the real answer is to get out the charts, probably unavailable to most of us, and compute all the numbers for the TODA, TORA, and ASDA. Compute Vmcg, Vcefs, Vrefusal, accelerate-go, etc. A 20 minute process, if you have the charts and not very useful in an airline environment.
GF