Perhaps you could do this with an ant external task that's triggered from your own custom Gradle task.
Section 2.8 of the user manual describes how to use ant tasks.

Are you suggesting a Gradle-specific wrapper for Ant's external task
or do you envision something even spiffier? If so, what would it look like?

What we should provide soon is an Ant plugin, which takes an Ant build.xml and provides all its targets via Gradle tasks. But the issue goes beyond this. It is about build integration. In the future Gradle should offer to choreograph a multiproject build consisting for example only of Ant and Maven builds, which are not aware of each other. But we haven't thought about details yet.

As announced on the Gradle blog we are planning to completely migrate issues from JIRA to GitHub.

We intend to prioritize issues that are actionable and impactful while working more closely with the community. Many of our JIRA issues are inactionable or irrelevant. We would like to request your help to ensure we can appropriately prioritize JIRA issues you’ve contributed to.

Please confirm that you still advocate for your JIRA issue before December 10th, 2016 by:

• Checking that your issues contain requisite context, impact, behaviors, and examples as described in our published guidelines.
• Leave a comment on the JIRA issue or open a new GitHub issue confirming that the above is complete.

We look forward to collaborating with you more closely on GitHub. Thank you for your contribution to Gradle!

Thanks again for reporting this issue. We haven't heard back from you after our inquiry from November 15th. We are closing this issue now. Please create an issue on GitHub if you still feel passionate about getting it resolved.