score:2
So I checked this out today, and the connection is feasible but not great: doable with a backpack, less so with a suitcase.
There's no infrastructure as such (bridge, tunnel, etc), but both stations have signage pointing the way and Google Maps's suggested route along Jl. Plaju appears walkable. Just don't try walking up the extremely narrow car ramp if coming in from the MRT!
July 2019 update: An Indonesian source tells me a connecting tunnel between BNI City, Sudirman and MRT Dukuh Atas is now open! Looking forward to checking this out on my next visit.