A lead-acid battery's internal resistance actually depends upon the state of the battery's charge, and the temperature. While you could use the drop in the battery's voltage to determine the current that it's sourcing, the calibration could vary greatly over the course of an event, and even over the course of a match.
Of course, now you're going to come back and tell me that that's what you do, and that it works fine, aren't you?
What we need to do is petition Innovation First to include a current monitoring output on their Victor speed controllers.