I don't want to give you false hope here, but we were able to get a Chef Mickey walk up for a party of 8 on Saturday, August 14th (evening before FD started). We had about a 30 minute wait and were seated around 7PM. (We had also tried for a walk up at Ohana and were told our wait would be anywhere from 10 minutes to 110 minutes - but that they would get us in.)
On Sunday, August 15th, we stopped in at Mama Melrose to try to move our ADR to earlier in the day, but were told they were completely booked up for the day. Later that night, my sister and BiL tried to make late evening ADRs for a party of 2 either in the MK or at a monorail resort restaurant and they were unable to. No walk-ups were available either. HOWEVER, many of these restaurants were willing to take large groups (of 8-10 ppl) on a walk up or last minute ADR basis.
So, is it possible? Yes Likely? No