Shadow on invisible model

[CasperH]

I hope someone can help me with the following, I have been at this for a few hours now with no luck. This side project is for a website, where an image is loaded by default in the background and models are loaded, always at the same position, on top of it. What I can't get right is that the shadows fall correctly on the vertical wall behind the model if I split the models in 2 images. The ground shadow is always OK.

As a reference, default-world.jpg is simple example of the "world".

There are two different sized/colored blocks that go over it (block-for-overlay.jpg and block2-for-overlay.jpg). However, their shadow only drops on the background as long as it is visible (block.210.jpg). I would like an image where the block has the correct shadow (as if it falls on both the floor and the wall) with alpha (transparency) but no wall model being visible (blocks-shadow-compare.jpg). Can anyone point me in the right direction? 

I searched/googled for the following, with various results: Keyshot, shadow, backplate, ground plane, back plane, ground material type etc. I think I have to use the ground material type, however when I do that the gray block always turns completely black and thats that.

If Keyshot could produce a shadow pass I could do some touching up in PS but it does not seem to be a feature.

Found some blog post comments, they refer to ground material type usage or Keyshot 4 and 5 having this feature, but no instructions.

Opposite of what I want, invisible object, but visible shadow: https://www.keyshot.com/forum/index.php?topic=8250.0 I have been able to reproduce this (see images).

Watchted this youtube video, but no "shadow layers" https://www.youtube.com/watch?v=5Zz9Kq2Pr1U

Found this forum page, but its basically a hack and only works if you need shadows on one plane, not on two, like I have: https://www.keyshot.com/forum/index.php?topic=5061.0