I dont see it being an issue. I'd have to do some research but I think the main difference between 8 and 9 speed systems isnt the thickness of the rear cogs or front chainrings but its the thickness of the rear spacers.

More thin rear spacers required thinner chains to prevent rub but I think was primarily accomplished via thinner side plates versus a decreased inside dimension. Think about it, Shimano 7 and 8 speed rear cogs are 1.8mm thick while the 9s cogs are 1.78mm thick.

Now, if we logicly think about this in relation to front chainrings and we assume Shimano's 9s ring is thinner its only going to be thinner by .02mm. Thats .0008". Use the 9 speed ring.