This was my plan for PBP and as a part of that, I actually strapped a light sleeping bag, changes of clothes and a small tarp to my Carradice for all of my training during July. The weight was ... there. But, I always assuaged myself by thinking that if I couldn't finish a 1200k because I had an extra pound on my bike, then I had no business being on the ride in the first place.

One thing I would advise for roadside camping is to be sure to also bring earplugs and an eye shade if you're a relatively light sleeper. Granted, one will usually be so tired on a 600k that sleeping soundly shouldn't be a worry, but the last thing you want is to have a deep REM cycle interrupted by a tractor-trailer roaring past the field or rest area that you're sleeping in.