Is there Ubuntu's analogue of snapshot.debian.org?

Unfortunately there is no current exact equivalent. However, Launchpad does keep all the historical data required, and you can access this directly.

For example, for the package hello:

• Go to the Launchpad package page: https://launchpad.net/ubuntu/+source/hello
• From here, you can View full publishing history.
• From this page you can work out exactly what version was published for a given release at a particular time you're interested in.
• Click on the version string. This gives you the information Launchpad holds relating to that particular version.
• You can download the sources for this particular version from this page.
• From here, click on your architecture in the Builds section on the right. The release stated here relates to the release the build was performed on, not necessarily the release the version was published in, so won't worry about the release stated.
• You can get download the debs from the "Built files" section at the bottom, and even see the build log.