Definitely a big market which many brands are missing out on, fully understand why though but surely in 2010 manufactures will see the potential market. Thought about this and putting your manufacturing head on id come up with an idea which has no extra costs either to printing costs or manufacturing costs.
Have two different sticker designs even for the slightest change and that's cost ? add something like mark ( ss stamp ) on the willow extra costs. Got a couple of suggestions so far which is about the stickers and not the bat willow, so keep the stickers exactly the same for made bats or stickers to be sold separately and cut a corner off them ? just a small corner !! or again keep stickers exactly the same and add a small hologram type sticker separate on top of the original stickers for branded manufactured bats.
The holograms made only for manufactured bats and printed in bulk all on the same sheet, costs would be very very little but profits would be large. This keeps it simple for people buying bats and genuine and non genuine can be identified quickly and easily by everyone.