[Phanx]

You don't need to make a whole new function, just check the callback and shift the args as needed:

function DoSomethingWhenResStarted(callback, targetUnit, targetGUID, casterUnit, casterGUID, endTime)
     local isMass = callback == "LibResInfo_MassResStarted"
     if isMass then
          targetUnit, targetGUID, casterUnit, casterGUID, endTime = nil, nil, targetUnit, targetGUID, casterUnit
     end
     -- do stuff

It would probably make more sense for LRI to change the order of args to better facilitate a single handler function, but that would break backwards compatibility; currently, addons that don't add the new Mass Res callbacks should continue to work as-is.

Edit:
The MassResStarted callback was missing the endTime variable; that's been fixed in r67.