Tracking an event that takes place on another site

Our use case requires logging an event that occurs on a different domain than the website being tracked. Think of it as a conversion that takes place on an external site.

Here's my first pass at how to do this, based loosely on this forum comment: http://clicky.com/forums/?id=6592#6600

- On the site, in javascript, create a "pseudo_session_id", and log it as custom data in clicky events
- Using a beacon (or similar), set a matching cookie for the external site's domain, with the same value
- When the conversion takes place on the external site:
- query the API for the "pseudo_session_id" and use that to retrieve the session_id
- use the clicky_log() function to log the event, using the session_id retrieved


So, my questions:

- Is there any way to enforce a 1:1 relationship between our "pseudo-session-id" and clicky's session_id? On what basis is click's session_id refreshed? Is there any way to detect this via JavaScript?
- Am I missing a simpler way to achieve this?

Notes:

- We control the external site, but on the tracked site we are limited to JavaScript.
- We can't use the user's IP address as the identifier in clicky_log(), because we know that the users of the site being tracked will often share IPs (different employees in the same company).