Michael Hatfield of the PMI offers an EVM tool that doesn't require a WBA or baselined plan.
"The traditional formula for calculating an EAC (EAC = BAC/CPI) can be algebraically reduced to dividing cumulative actual costs by cumulative percent complete. That's right, we're talking two date elements, easily collected." Check this out.
Do you use EAC in your PM status reports? If so, do you use this model?