Hey Ceptoplex - Product Manager for Extensions here!
Requesting a viewer share some information to power your extension is acceptable - your example of an extension requesting the viewer’s account name in text within the extension would be fine. The oauth restriction is specifically for permission-granting flows for offsite services.
Our policies are designed to protect our users. This is an area where we want to start out conservative and once we have a better understanding of how people are using extensions, we can revisit the details of the policy. Please do let us know if the policy or product is not allowing you to do something that you think would be really cool - extensions are your product, and we want it to work for you!