In AGS I could define the walk-behind area by filling in the solid part of the railing (it even allows importing masks for this). How would I do it in WME?
Simply: the new layer would have transparent areas

In AGS you fill solid areas, in WME you fill transparent areas.
This has been discussed several times before by people used to AGS's approach. Let me sum it up:
1) I really don't see a big difference between painting the walk-behind AGS region and cutting out part of the background a painting the transparent areas. It's basically exactly the same work.
2) The layer approach allows you to use an alpha-channel to have antialiased edges/trasparency on the obstructing object. I'm not sure it's possible in AGS
3) Performance! The layer approach can take full advantage of hardware-accelerated image drawing. The AGS approach can not (at least not easily, I believe it would be technically solvable though).
4) If you use pre-rendered backgrounds, you can render those separate objects "for free", simply by choosing which 3D objects to render. Thanks to alpha-channel, those rendered objects already have the transparency included, i.e. no more hand-painting.