I haven't tried it, but I guess that you need to define the arrow slots in the info model. To see how an info model looks like open a siege tower info file and keep Jerome's post at hand. You will then start to understand how it fits.

To test my theory I used a siege tower info file as a base, kept a single platform plus the Scene Root. Then I placed at the same height and with the same width and length as the top of the wall. Checked wether the front edge was at the right place (I guessed the outer side) and CSCH or whatever it should be. I got it ingame and while there was 1 soldier getting stuck it did work.

I am the moment busy with other things. This thread was mostly intented to show that the interaction with walls and buildings can be edited. I am far from being an expert on this issue.