Easier to do if/when building new at C/D, but the flow would be like it is now except that when you get off plane into segregated corridor you would walk to train where there's a segregated station (connecting pax would go to midfield customs). Trains stopping there would stop only at arrivals hall but would otherwise use the same tracks/tunnel.

As it is right now, you walk to the end of the C concourse (near C1) to get onto moonbuggy. You would just have people walk in the other direction (towards train). Retrofitting A/B would be harder because they didn't design around this plan, but concept would be the same. They might have to expand the station underground to create a separate platform, but you'd only need one side because flow is only one direction. (And maybe there's even room to use one end of one existing platform for this at B.

That all said, might be easier to install some of those international-style jet bridges where you walk up or down depending on whether you're international/domestic/Schengen and have arriving int'l pax exit jet bridge and go down an escalator to ramp level and get on Cobuses to the IAB, which could be reconfigured for a ground level entrance. At least then the moonbuggies can be retired.