This will work fine, except there is no RF out on the 6412. - only composite and S-video.
Yow will see all channels on the SD TV including HD (will be letterboxed), but you will not see any menus - those are only sent out the Component (or DVI). No downconverter needed.
DVR and VOD also work fine this way except no menus.
I have this exact setup - a small SD tv in my kitchen that just mirrors the HD tv in the living room, so the person in the kitchen can still watch the same show - just in SD for now. It is one big room, but you just can't see the main tv from the kitchen, hence the SD TV mirroring. I want to eventually get a 17 inch LCD HD panel in the kitchen, and then use DVI on the main tv, and component to the kitchen to achieve HD mirroring.