I was looking at the plans for the Victoria Transport Interchange* development, when I found this diagram showing where the platforms for the Chelsea-Hackney Line will be (the grey bits on the left), with interchange passageways shown and everything!
The southbound platform will be directly under Buckingham Palace Road, the northbound platform a bit off to the west, and there'll be a new entrance on the north side of Bressenden Place (a long way from Victoria station, but handy for Buckingham Palace), and an interchange passageway across to the District Line ticket hall, and another to the Victoria Line platforms.
Sadly, it's all fictional:
No detailed plans yet exist for this scheme but the Victoria Transport Interchange proposals incorporate a safeguarding zone anticipating the station being located below Buckingham Palace Road.And the redevelopment scheme itself is looking shaky.
If you still want to take a look, the diagram is on page 18 of this 108 MB PDF, and pops up again on page 50, and there are some cross sections at the back.
(* Ignore the name - it's actually just office blocks in the area north of the station, its major effect on transport being "removal of the bus station from in front of the station and dispersal of the bus stops to the surrounding streets")