The passes and files export is managed by the Octane core. There is nothing on the plugin side affecting them.
The render options things that can affect the output file and perhaps the Z pass are the file format (especially if it is tonemapped or not), the compression format, the EXR half precision, etc. You should check if these settings are the same in C4D and Houdini.
From the RenderTarget point of view, be aware that there is a "Z-depth max" parameter that can also affect the Z pass output. You should also check if this parameter is the same in both plugins.
I can't figure out anything else that could explain why the z pass is not the same in all the plugins. If all fails, perhaps you should export both scenes to the standalone to check what could be the difference in the render passes node.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini