Ask HN: How do you manage technical scope?

I'm charged with producing a technical architecture for a big enterprise web app. The scope of the application involves lots of moving parts which end up making a consumer financial services website with including purchasing stocks/financial products online.

Nothing too out of the ordinary apart from lots of SOA/enterprise systems integration complexity.

The biggest challenge I face is not really how I would build this, but how the client would like it built. They want a custom CMS to control almost every aspect of how the system business logic works. This will naturally add huge overhead to the build and is making it very hard to quantify. Red flags are popping up everywhere with regard to managing scope. This is looking like a big multi-year project.

I'm curious as to how others deal with this? Any suggestions greatly appreciated.