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For the sake of anyone coming across this with a similar problem, turns out the tutorial on Feature Matching and Homography was a pretty good start; my task appears to be a case of that. Computing descriptors with BRISK and using those to train a FLANN-based matcher using LSH absolutely nails it. (BRISK seems to be significantly more accurate than ORB, correctly finds even the most difficult query images ORB couldn't match, but is does push my limits of performance and memory usage.)

Thanks to @berak for the the reply he later deleted -- I suspect you noticed my condition that the query images may be enlarged from the original meant template matching wouldn't quite work but that got me looking in the right direction anyway.