It depends how the rigging is done. If its is creating votes ex nihilo then indeed you would need nearly 100% of the absentee ballots, but such rigging creates the risk of being detected if the number of votes is greater than that of the registered voters. However, if the rigging is by electronic switching of votes (e.g. a legit vote for a Democrat is recorded as one for a Repug), that all you need is the absentee ballots of those intended to vote Democratic.
In any case, an unusually high number of absentee ballots may subvert the push for electronic voting machines because it will greatly increase the cost of tallying and thus make this option less attractive to the states, especially that most of the m face a budget crisis.
Wojtek