In Photoshop, you can easily add additional space to an image by dragging the crop rectangle outside the current canvas. This would be really useful in Lightroom when some additional space is needed to get the right crop (very common in particular when doing photo books). Some options for handling the fill is all that would be needed.
Agreed Rikk. Mostly I'm wanting this feature for situations where I don't need anything more than a solid color fill and it's really overkill to have to open PS just to add that. Having this feature (along with the ability to set a transparent background color) would save me a lot of extra time in my post processing.