GET request variables (variable entry points)

I’m writing a plugin that involves visitors going off-site and then returning.

Currently it is a simple content_filter grabbing _POST variables via a series of forms up to the point where they leave the site. I then need them to come back with an (equivalent) session_id so I can continue their process.

I understand that _GET variables are not available, but this must be a reasonably common problem – is there a standard way of tackling this problem?

The initial spec for the plugin includes functioning without the use of cookies.