I'm also on the 400 BP and also intend breaking the ride up with an overnight camp, maybe 250 Km on day 1 with a leisurely 150 Km on day 2. I have never bivvied before, but quite fancy it for this adventure, as I feel it is more in keeping with the ethos of the event. My main concern is actually finding somewhere that I could actually do this that is practical in terms of privacy and relative comfort, and not get to the stage where I'm riding in the dark and not being able to find anywhere.
Any pointers in this respect.
It is a law universally accepted that you will cycle past a bivvy spot that seems ok, but perhaps not the best, but not find anything anywhere near as good as the one you cycled past, and end up bivviing in a slightly sub optimal location down the road because you've hit your limit and just have to stop. Upon resuming your ride in the morning, you'll find 500m down the road the most perfect bivvi location you could imagine... Thus is the law of bivviing...
I've bivvi'd with a number of different bivvi bags in varying conditions in various countries. I had to learn best practice the hard way... with practice... And aside from a nocturnal encounter with a wild boar in Spain, not had any major issues when bivviing. These days I prefer to sleep outside in a bivvi bag to a tent. Laying there watching the stars go by as I drift off is just an amazing experience. I highly recommend it.
Thus, Other than the above, the rules of bivvying are simple:
1) don't breathe into the bivvi bag.
2) don't breathe into the bivvi bag, technically this is the same as point 1, but it's so important it's worth mentioning twice. There are some bivvi bags that are sufficiently breathable that you can breathe inside them, but they are few and far between and cost the gdp of a small developing nation.
3) Insulation from the ground is really important, don't skimp on your sleep matt
4) Whilst the bivvi bag is wind proof, this just means that the wind won't go straight through to your skin and chill you that way, it doesn't stop the bag from getting cooled by the wind blowing on the outside increasing heat loss. Pick a spot out the wind if you can.
5) Even if you relieve yourself before crawling into the bivvi bag, the laws of the universe mean that even if you're dehydrated, the moment you've crawled into the bivvi bag the first time of the night, you'll need to pee about 5 minutes later and have to go through getting out and in to the bivvi bag all over again, accept this, it's not worth the fight. This is even more so if you have a thick down bag, with a liner inside it as well...
6) It's supposed to be fun, no really it is...
7) Check your shoes for slugs before putting them on (you only ever make this mistake once...), if you have enough space, putting your shoes in a plastic bag in the foot of your sleeping bag keeps them warm and keeps the slugs out.
Hope this helps.
J