Sandy is right ... I forgot to mention that the DNGs have color profiles embedded, so the DNGs themselves are not actually untagged. My use of the word "untagged" was meant in terms of the color transform applied (i.e., there isn't any).
The noise reduction algorithms in Lr 4 Beta are aware of the relationship between the image size of a downsampled, lossy compressed DNG (e.g., 10 MP) and the full-resolution original (e.g., 36 MP) and can therefore adjust its internal parameters accordingly. This happens automatically, without any specific action needed from the user. You can expect noise reduction to be extremely effective on high ISO, downsampled, lossy compressed DNGs.
Hope this helps,
If you are converting a lossy compressed DNG to another lossy compressed DNG of the same image dimensions (i.e., no downsampling), then there is no loss of quality. This is because we recognize this special case and simply copy over the compressed data stream as-is (verbatim). Thus, we do not decompress & recompress, which would lose quality.
Of course, if you are generating a lossy compressed DNG of another image size (e.g., going from 10 MP to 4 MP), then of course the image characteristics will change because the image is getting smaller ...