I've installed Google Portable 24 beta from PortableApps.com. At the bottom of this page you may find installers for Google Chrome Portable Beta 24.0 and Dev 25.0.
After installation I opened the About page and the version is
24.0.1312.40 without a beta suffix.
This is confirmed by the problem I encounter when I try to install extensions utilizing new features: Chrome outputs a notification under extension's name, that it "requires Google Chrome beta channel or newer, and this is the stable channel."
The question is: how can I download a portable Chrome version which would be a beta (or development release) indeed?