And it would be very costly...From OKW you would have to cross a major roadway footbridge, then cross the parking lot to the main entrance. I just don't see WDW adding a new entry gate to EPCOT through the backstage area just for OKW and SSR, which would still be expensive but less distance than to the main entry.
The only way I see this happening were if it were part of a larger project, including some sort of tracked transit system (like a peoplemover or automated rail) connecting OKW, SSR, DtD, TL and EPCOT. However, this then would be like opening the DtD parking lot for theme ark acces to off site visitors, more trouble than such a project would be worth.