Angled Rectangular Select (or rhombus select I guess) - possible?

[Ofnuts]

I don't completely understand what you want to do but:

1. You can apply all transform tools to the selection mask using "Transform: Selection":    
   
2. You can save selections (Select > Save to channel) and combine (add/subtract/intersect) saved selections (see here for more info)
   

For instance to create a rectangle selection with a rhombus hole in it:

   

Note: after the 45° rotation, the selection seems to vanish, because the "ants" cannot show on  45° lines. You have to work on faith, or check the selection by other means, such as the quick mask or the selection editor.