Ability to drill through without logging into the application

You're effectively asking for a way to publicly "publish" data.

What you could do is, build a book that saves reports as files in a location that you then make publicly accessible. Unauthenticated users would then be able to retrieve those files and read the related data.

Another, much more complicated approach, is to build a completely different interface that, in background, uses the OneStream API, or the OneStream REST API, to log on with a predetermined "service" user, and retrieve data to display publicly. This could be a web or desktop app.

This said, security controls exist for a reason, so one should be very careful when building such solutions. There is a significant risk of leaking very sensitive financial data.

It will be much easier to simply add a few more users to the application and allow them to see the data they're entitled to see.

Can you confirm if you're wanting to drill FROM a cubeview/dashboard to another (non OS?) application or if its the opposite?

Apologies for the delay in responding. I was waiting for some clarification on the requirement.
We need an extensible document which has the capability to expand and collapse the members used in rows. For example, if we use Entity in rows, users should be able to expand the top parent to view the child member data and collapse it back. Sorry for the confusion. Thanks