Currently with the notification API you allow users to include various custom values using "cv1", "cv2" etc. However, I'm implementing a solution that could have a varied number of custom values and so I can't reliably create a notification url using your existing tokens to account for all the possible numbers of custom values.
The solution I'm using right is just to use your built-in hooks in the payment processing to capture the custom vars and do my processing there, however, it was a real pain to discover all the variables available to me and even with the hooks I'm concerned that future updates to s2member will create a maintenance headache for me.
A better solution would be if you can provide a token for sending the entire "custom" string via the notification api url (i.e %%custom%%), then I can just do my processing on the script I have at the url.
I'd LOVE it if you could implement this in the next release. It wouldn't be hard to add.