Well to give everyone the answer here to BigFurry's question, because I imagine there are others who would like to know, and information is key... What he is specifically trying to accomplish can be done. We have a few wiki articles describing the process with NATS for you to review at your leisure.
An excerpt from the NATS4 Additional Tracking page on our wiki:
"In NATS 4.1 (version 4.1.7.1 and above), you have the ability to allow your affiliates to add additional passthrough variables in their linkcodes that will get stored with a member's subscription record. These variables become available on postbacks. In addition to these passthroughs, methods for forcing the join option to be used on the join form and changing the default join option have also been added. These additional tracking features can only be used if you allow them within their "Edit Affiliate Admin Settings" page. The two settings are called "Subscription Passthrough Variables in Linkcodes", and "Force Join Option in Linkcodes". Once you enable the feature for their account, they can add a "nats_at" array with the additional variables to the end of a link code"
See the full resource here: NATS4 Additional Tracking
An excerpt from the NATS4 Affiliate Postback page on our wiki:
"NATS4 has to ability to post detailed member information to your affiliates based on any basic action the member takes through NATS. This is made possible by using a backend POST, which is based on the basic actions the member takes in NATS. For other postback examples, please check this article: NATS4 Postbacks and Post URLs
To use this feature, you must first activate it in the Configuration Admin. When it is activated, your affiliates will be able to receive detailed information on what actions their members are taking"
See the full resource here: NATS4 Affiliate Postback