It would be useful to develop a custom field that integrates with Confluence. The custom field would allow a user to select a Confluence page (and in the future - multiple pages), from one or more confluence instances.

In the setup for the field - you can select which Confluence instances you want to integrate with, and assign them a unique id (eg. cac = http://confluence.atlassian.com/). Then if you use a text area - you can specify 'Confluence instance / space name / page name'

eg: 'cac/DOC/Documentation+Home'

JIRA would then save the 'id' of that page in the JIRA database.

I expect that most users would want to search for pages - we would need a popup (using the webwork plugin) that would enable users to search the confluence instance for the page that they want to link to. This would be using the Confluence remote API.

We also need to be able to search for pages that are linked to a particular confluence page. This would enable a macro to be written in JIRA that allows you to show all the JIRA issues that are linked to a particular Confluence page. (how cool would that be for feature requests!).

A longer term solution would be if you can search across both instances!