Hmmmm, whilst I like the thought of the OSB (i.e. something nice to walk on) I don't like the extra cost.
I might just keep to battons and make a single crawl board e.g. piece of OSB with a 'lip' so it sits over a batton - gives me a nice surface to walk on and not worry about putting my foot through the membrane or breaking my leg. Roof is 30deg incline so sounds safe too me.....