There are some issues with this, in some cases using a
DAS is the same as having a wheelchair as far as ride operations go, this is particukapar the case where there are a limited number of wheelchairs allowed on the ride at a time. This means that a DAS user might be able to free up a wheelchair space for someone actually in a wheelchair by using FP instead. Additionally, some attractions, such as Pirates (DL) and Space (DL Version) take more time when you return than the standby line. And almost always the time you waited plus the time it takes when you return is always longer than the standby line.
Obviously Pirates can sometimes be worked around, but if the DAS holder needs more time to get on and off or is using a wheelchair, they have no choice.
But no matter what because the wait for the return time plus the amount of time waiting when you return is always love get that the standby line. Having had a lot of conversations about the DAS system with high level executives, they have told me that these were the main reasons that they decided to not only allow FP use with the DAS, but to encourage it.
So to your thought, not, that wouldn't work and wouldn't actually keep things fair.
Getting a return time from
MDE would be no different than going to each attractions to get a return time, other than less walking and the ability to reduce staffing levels to a certain degree and allow Disney to save some money.