The Clover Coverage Gadget is apparently part of the Bamboo plugin that is bundled by default with JIRA. It Studio is simply doesn't work. The gadgets don't show up by default in the gadget directory.

• According to the docs, it requires JIRA to be registered as an OAuth consumer in Bamboo, which isn't set up by default (and the link to manage oauth consumers is hidden).
• The gadget URL http://$host.jira.com/builds/rest/gadgets/1.0/g/com.atlassian.bamboo.gadgets/status/cloverCoverage.xml resolves and returns a valid-looking descriptor (both externally and on the Studio host), but trying to register it with the dashboard causes JIRA to return 400 error in response to the POST, stating Could not parse gadget spec at: http://$host.jira.com/builds/rest/gadgets/1.0/g/com.atlassian.bamboo.gadgets/status/cloverCoverage.xml. I can register non-Studio Bamboo gadgets this way (although I haven't yet seen one fully working).