I have been having a play with this problem this morning and have sort of come to a conclusion over the best way to approach this. I think ultimately the best method is that suggested by John R. I give my reasons for this:
1. The xclip method would be OK if you can make sure you can window a boundary around all the landscape objects and not miss anything. I also assume that once you've clipped the objects you can't get back the clipped area if the design of the shape of the building in front changes. So you would have to reinstate anything lost.
2. The layer order method didn't seem to work for me. Maybe I'm doing something wrong there but I looked in my TC trainging guides and there is nothing about layer order there to enlighten me
3. The join polyline method doesn't seem to work very well as I don't seem to be able to easily and quickly use the join polyline tool on an object with many individual lines.
4. John R's method of using a boundary with a solid hatch works if the colours are set right for the printing. Another advantage of this method is that if the design of the building is changed then you just have to alter the boundary of the hatch and the landscape objects reappear. You can also have multiple, overlaid landscape objects with some behind the building and some in front as they may be on site (see attached PDF). To work on the drawing in model space you just turn off the layers for the solid hatch and the landscape features as you would do normally anyway.
So, in conclusion it would appear that John has won the teddy bear for the best method so far of achieving this. Unless, of course, someone else has a better idea