Seems like a good plan (excluding the helmet/seatpost), provided you can attach the U lock to a fixed object then through your rear wheel+frame. I suppose I just don't like the idea of leaving my helmet out, even locked up-some crackhead bob may just try and pull it/cut it off. I'd guess 4' of cable will work when excluding the seat. If it looks as though the U lock won't fit, or due to rack crowding, might only work sometimes-you may be better off with kryptonite hardened chain to the frame/object/wheel, and U through the other wheel (thats my setup at work).

For the seat you can use old bike chain in a loop from a rail to the frame (wrapped in an old tube or electrical tape). I use a small cable lock with a combination (cheapo ~4-5$) looped between the seat rails+frame a couple times. Also comes in handy when I stop at a store (in a low crime area) on my way home and I left my U lock at work.

--Earlier this week I saw a rather nice/new commuter bike locked to a rack with a new york chain by the FRONT WHEEL ONLY! I didn't have a chance to go back & leave a note later in the shift, nor have I seen this bike since-but thankfully there isn't just a wheel there now!