The passage from underpass to central hallway described in the rejected idea section is used in the CSGO map de_shortdust: https://youtu.be/rrXeowmXLMc?t=6m6s Of course, the gameplay mode used with that map is different from one de_dust is designed for.
And it exists in the CS:GO version of de_dust as well, though it's on the 'off' side of underpass. You then have to cross the ledge above Underpass into the hallway.
