Any chance of being able to generate shadow maps larger than 640x480. Perphaps allowing ‘Auto-generate Depth Map’ option to render arbitrary size??

No, I don't see that happening.
Shadow map generation is a very important step in production taking considerable time, especially if it is deep shadpw maps.

Houdini 8 now has deep shadows so use these for shadow maps. You can get higher quality results with deep shadows with a lower res than regular shadow maps at the same resolution.