Registration Pt headache

I'm having a headache designing my 3d game engine... I finally ALMOST got the first type of object done, but some freaking Vectors question is bugging me.

Here's what i'm trying to do. Shifting the registration pt of an image in 2D is easy. Just move the object.

Now, a billboard. It constantly faces the camera. Don't ask how, it's some internal directx thingy *shrugs*.

Okay, so i shift the billboard in the normal view, which is ground level view. I shift it upwards and sideways. Just like a 2d image thus far.

But what if i change my viewing perspective from a ground level to a top-level? A billboard always faces the camera, but does the shift done earlier [sideways and upwards] look the same in a top-level view? In the top level view, does the billboard looks like it was shifted to upwards and sideways as well?

Here's a picture.

The way to solve the above example would be, in the ground view, to move the billboard LEFT and INTO the page instead of LEFT and UP. Then, when viewed from the top view, it would seem as if the billboard has been moved LEFT and UP, which is correcting for the registration pt.

But... 'course it's not THAT easy. I'm NOT using a top-down view. I'm using an ISOMETRIC view. Think of it like Red Alert 2 style. There is no depth. No matter how far things are into the page, they look the same size.

And to make things more complicated, i have no idea how an Isometric perspective matrix works. One need a perspective matrix to tweak the camera viewports. Yes i have a matrix, but i dun noe how my matrix works. It just ... works.

What am i gonna do?! Am i just scrapping the registration pt feature? If i do so i'm forcing a complete adherence to same size images, with center as registration pt.

Or i could manipulate the image and fill it up with empty space, thus changing the size of the image and making the registration pt the center without needing to move the image physically.

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