Download data from Amazon S3
One option to make the data available is to store them as a public dataset on AWS S3. https://aws.amazon.com/public-datasets/
It would require some development (a new plugin?) on the XNAT side in order to create the access for a user who has already signed the data use terms.
The HCP is doing so using the AWS SDK (another alternative is jClouds) and most of the code is available in the custom XNAT dbconnectome source code: xdat_release_cust/src/java/org/nrg/xnat/restlet/extensions/AmazonRestlet.java
Other interesting source files can be found by searching for "awsAccessKey" in their source tree.