That can't work because Google does content-based filtering. They blame the forwarder for any spam or anything forwarded that's spam-like, and there's no way to designate a source as a legitimate (that is, don't blame it) forwarder.

I use 33mail with gmail as the actual destination.

I do this using forwardemail.net. If a particular address gets sold and is being spammed, it's trivial to shut it down so it won't forward anymore.

Receiving is easy, sending is hard. That's why disposable services let you read the mail in their GUI instead of forwarding them.

AnonAddy is great. Though I'd like to know if there's anything open source like it, for the safety and longevity guarantees.

AnonAddy is open source...

Huh, it sure is. Thanks! :D