Dan mentioned that ending in an interview about three months ago, can't remember what website, he may have linked to it on his or The Chinese Room's twitter. He said something like; it would turn out that the machine's temple was directly under the sewer and Mandus would have to destroy the ceiling supports to flood the chamber and "literally drown himself and the machine in shit".
Even though it's a far more fitting ending, they didn't use it apparently because they were technologically limited.
Doesn't sound hard to make though, as surely all they would have had to do is take a few of the gushing blood assets from the earlier sections (around the room waist deep with blood), recolour it brown, re-size them, put them near the distant chamber walls and have the chamber slowly fill with sewer water from... well... the sewer level. Add a slightly edited version of the last monologue as he drowns, fade to black, cue "When I Was A Laddy", roll credits.
Awesome.