No, there is no option against 6 Demo-men, spamming a corner with 'nades other than the entire team rushes together, 8 people die instantly, 2-3 die after and whoever's left alive might kill enough people for the few to push the cart around the corner. Even an "uber" is useless if there's one smart pyro in that Demo-Spam team.
Welp, you've played a lot more TF2 than I have, but all I can say is I primarily play on a 24/7 hoodoo server, and have never had that happen. Usually when they can't get past that last turn, it's because of sentries, not demos. Every time there's a ton of demos on the other team trying to protect a point (in this or any other map), they all seem to blow their whole wad on the first scout to come running, then the rest of the team flows through after that one overkill. Also, overfocusing on stickybombing one chokepoint is just asking for 2 or 3 spies to sneak through/come around the flanks and murder half or more of your team before you even know what's happening.
And, as I said, the first part of hoodoo has 2 flanking path options at the final turn, the second has three including an absolutely
cherry sniper position, and the third has easy access over the wall to attack from behind from an elevated position.
Sentries aside, 9 times out of the remaining 10 when I've seen a team that couldn't seal the deal on hoodoo, it was because red switched to at least 2 pairs of medics and heavies, usually 3. Can't airblast 'em all.