A full depth sliding rack can be loaded to ~48 cm from the boot floor and slide out under the MF. However, the front part (the section between the back of the bench and the MF) can only be stacked to ~30 cm above the boot floor for the bed to be made up.
To pull out the sliding tray, either stuff above the front of the pull out will need to be unpacked or you will need a board ~150cm by 40cm resting on the wheel arch trim to support luggage above. At 1.5m long it will need to be a fairly rigid board to minimise bending in the middle (or stuff above fairly light).
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If I do end up making a full length tray, I think I would try to overcome this issue by adapting your idea a little and making up a folding bridge to fit between the back of the bench seat and the front of the MF. The basic concept could involve the board you mentioned spanning the gap but with a hinged leg of the same width at each end. The hinged legs would of course stand in the free area outside of the tray sliders and should be able to be locked in the down position to give rigidity when the bridge is in use. The height of the top surface of the bridge would need to be no more than the 30 cm measurement. The bridge could be folded flat when not required and shouldn't take up much space.
As you suggested, such a contraption would enable both a long tray to be used at the same time as the downstairs bed and would enable it to be much easier to reach items stored on top of the bridge.
However, I need to give the whole tray thing much more thought before proceeding.