The book doesn't flesh out this detail either. Since each district is polite enough to have at least one male and female previous victor, alternate scenarios aren't discussed.
However, that does beg the question of why the Quarter Quell rule seems a bit unreliable.
Along those lines, I got the distinct impression that the "sealed envelope" rule is subject to being created ad hoc by Snow. Creating a new sealed envelope with a new rule would be quite simple. For all we know, this may have been the actual intent for the Quarter Quells -- not to pre-fabricate special rules, but rather to allow for rulers to create their own as needed, in pursuit of maintaining control over the districts.
The major weakness with this reading is that the Quarter Quell only takes place every 25 years. As this is the 75th year, there have been two Quarter Quells already, with an established cycle of 25 years. That is to say, even if Snow used the Quarter Quell to inject his own custom rule, he was extremely fortunate that it was the 75th year and not virtually any other year.