Use a custom field to specify sprints in Greenhopper, rather than FixForVersion

We need a way to specify the tasks in a particular sprint in Greenhopper other than using the FixForVersion.

Because of Greenhopper, we need to use Fix Version to record both release version for a ticket (5.1, 5.2, etc.) and sprint number (Sprint 13, 14, 15, 16, etc., all of which are bundled into release 5.1, for example). We use Jira across the enterprise by the support team, product team, engineering team and sales team. All teams need to have access to the Fix Version field (sales and support suggest versions when creating tickets, product team manages what features go in what sprints, engineering team places tickets into sprints). When another team selects an item in the Fix Version select box, they usually accidentally erase the sprint selection, because of the way a select operates. Alternatively, they sometimes think they need to select a sprint. Our Scrummaster spends about 30 minutes a day correcting these mistakes. This was the major con in our purchase decision regarding Greenhopper.

Ideally we'd like to use a custom field (from Jira) to designate the sprint that each ticket is in. That way, we could set up access (permissions) to both items appropriately.