One of the nice features of JPEG 2000 is that it's easy to downres an image -- just feed out fewer bytes.
Is there a similar way to scale the image, such that I could send a thumbnail using bytes in the full-size file, without resampling?
No, to resize you will still have to read the original size, resample to the new size, and then recompress.
You can write your file such that it has layers with powers-of-2 sizes, with a very small relative incremental increase in size, and extract those layers.