Nick, Script Runner is an excellent choice as far as customizing JIRA goes. It has a lot of wide-spread power.
In order to achieve this goal of yours, you need to customize JIRA. You can use the plug-in API to add an add-on that does this, the remote API to script this, or you can purchase Script Runner so you have a platform in which to make these changes.
There's lots of ways to accomplish this.
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