Even though you CAN park in the overflow lot, be aware that depending upon the time of year, the Overflow (and even the Overflow of the Overflow) can get very crowded.
There is always the alternative to park at the TTC (after all, there is no cost for guests who are staying on property) and then use the other means of transportation as required. Inconvenient? Yup, but depending up on the situation it might be the only viable option. All that said, though, I think the need for it is rare and would only really be required in extreme cases (Perhaps something like New Years Day when I hear it can get REAL crowded all around WDW. )