From what I know of the LUL signalling and speed limits their boards are different to standard BR boards, and also the speed limit is active when the FRONT of the train has passed the board rather than the rear, which is true on BR Given the full route including overground sections has been promised, are we to assume these two signalling systems have been recreated, and has the issue of "front passing speed limits" also been recreated on the appropriate sections, with the overground remaining as BR standard?
If the route is going to be accurate, I believe London Overground follows how the Underground do things for that section of line.
This shouldn't be very complicated to implement. The LUL trains have a standard train length, so they can place boards in the tunnel at appropriate points to indicate that your whole train has passed the SR/TSR. Same thing applies in TSW2. It doesn't need any extra special programming.... you simply place the physical board (that you see on the track) one train length ahead of the "soft" speed limit (that you see on the HUD). It should be a non-issue.