Interesting trek your on. I think your major challenges are 2 things. Automation of Podio memberships and the fluid display of Podio content, outside of podio.
1 - Getting someone to sign up on your site, and adding them to your Podio account automatically via API. Plus, restrictions within Podio are not controllable on a micro-level, so content is more open and available.
2 - Once you have accomplished #1, your users will have to be in Podio to obtain, read, view or contribute information. What ever you are looking to get OUT of Podio, will take place IN Podio.
The most interactions we have achieved between the client and ourselves, is proofing graphic design work via Dropbox, and webforms that bring user contributed data directly into an app within Podio, where it stays. The Webforms are the only piece of Podio that I know of that can have CSS attached to it, and be embedded within a webpage.
There is a third party app for handling monetized transactions that can be integrated, but nothing that I've seen would integrate three way with a website, or run automatically. Everything I've seen is manually inputted and controlled.
Good Luck, Ill be monitoring this conversation. We are involved in some projects that could take 200% advantage of your item list above.