Have a look at this:
http://social.biowar...m/project/8951/
The general concept is to put invisible walkmesh in a tile. When useable, display a suitable placeable over it. When not in use, block it with invisible walls, optionally displaying a broken / closed placeable over it. Placeable display methods include open / close / activate / deactivate, cutscene invisibility (working around the hard-bake bug), or good old spawn & destroy.
Animations are highly recommended, because the timing is so precise that you can synchronise other effects perfectly. For example, I have a chase at sea; the hostile ship pulls alongside the player's vessel, its crew becoming visible at the last moment (of course, they have been standing on the invisible tile all along).
If you don't want to mess with tiles, you can still do the above, as long as the walkway is on level ground. CEP placeable doors work that way, for example.
Dynamic placeables don't render at a distance, of course, but that's business-as-usual.