Hi Molly, progressive profiling should work because it's just basing on whether or not the information is known, and not actually passing sensitive information over.
There are a couple of recent threads on this topic:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PqCBAA0
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PrtgAAC