Outbound SSO Suitelet - can we pass customer data to the IFRAME?

I have a Suitelet for our Customer Center, wrapping an externally-hosted PHP script. The entire outbound Single-Sign On works, and everything renders correctly. In the PHP code, I need to check that certain custom fields on the customer record are set and act accordingly. (The PHP script is rendering a remote server filesystem tree, hiding certain items depending on customer record fields.)

What's the best practice for passing data from a Suitelet into the IFrame where the PHP script runs?

Here are the issues I'm getting stuck on:

nlapiGetContext() doesn't provide customer information: the context contains the unique entity ID, but no other direct access to any of the current customer's record. So I have to do a nlapiLookupField() to get more customer record fields... but before that works...