We've found more cost-effective ways to achieve this, and that's been funded by a customer. So this is effectively achieved.

It still uses popups, but that's fine -- we've provided popup-blocker instructions, which we auto-link-to if the IM popup is blocked. Real popups are more suitable than overlays for this rare case - they work more like native apps (as the popups now persist even if the main site is closed, and you can have many open at once and still use the site).