Sounds like something Ezio could easily do -- adapt Rietveld's upload.py to a Roundup extension that submits attachments as patches, adds people on nosy to Rietveld CC, &c.
That may not be so easy - you'll have to authenticate to Rietveld from Roundup.
The other way 'round actually works: if you put email@example.com into the reviewer list when uploading to Rietveld, Rietveld changes get automatically posted to the tracker. If you then also chose a sensible title (ie. one that includes [issueXXX]), Roundup will be able to associate it with the right issue.