Billboarding on a single axis
I think the title roughly describes what I want to do.
As I understand, billboarding allows you to make an object always face the camera.
But what I'm trying to do is have my 'billboard' stick up from the model and have the X and Y axes constrained to normal perspective.
That probably makes no sense whatsoever, so have a picture instead:
imgur.com/z3EFPnX (apparently I'm not allowed to post images or links)
This example shows a rectangle on a plane with one side marked by a red ruler. This ruler will always stick up from the model so it's always visible, whether viewing top-down, or side-on.
But I don't know how I should go about trying to do this.