DAS does actually work for shows, at least at WDW
My family has used it for Festival of the Lion King, Nemo, Frozen Sing-a-Long, Indiana Jones, Bird Show at AK and the night time Rivers of Light show at AK while it was in existence.
DAS Return Time for shows differs from DAS Return Times for rides in several ways.
1) The show DAS Return Time has a window with beginning time
and an end time
2) The show DAS Return Time is for a specific show, so if you come after the end time, you probably won’t get into the show
3) if you need wheelchair/
ECV accessible seating, it may be all gone if you come toward the end of the window
My family had the same past experience with using DAS for Fantasmic as HopperFan reported. It allowed entry using the Fastpass entrance, but not specific seating area.
We needed wheelchair seating for one wheelchair and were directed to an accessible spot in the back row, with space for one companion next to the wheelchair. The rest of our group were directed to sit in the row below. The entire back row was wheelchair/mobility device seating, but a section of the back row was part of the Fantasmic Dining Package.
No one knows how Fantasmic will work now, so I agree with the others that booking a Fantasmic Dining Package might be the most choice if you feel seeing it is important to you