Can you fix the substrate at an oblique angle? You'll have to manipulate the substrate holder, modify the setup a little bit, but I don't see why not.

Wedge something under your substrate. If you want to heat it, use something thermally conductive, like Al foil, to achieve your desired incidence angle. Additionally, you may machine a small shim to achieve the angle you desire. If you want to maintain a UHV condition, make sure your shim does not outgas as your bake out temperature. 

od course you can, change optical geometry or change the position of your target and sample holder