I would like to use google's 'search by image' feature to search a defined set of images (rather than the entire web) for matches. Has anyone done this previously or do they know if it is possible?
Yes, this is possible.
If you enter the image url, the search will be performed directly, however afterwards you can just type in additional keywords, or for example a domain.
If you want to search in a set of images that typically cannot be found by google, then it will probably not be possible.