How can I batch convert multiple folders of images to PNG8?

I've got 4500+ PNG 24 images in 20+ folders that I wish to reduce the size of by converting them to PNG 8. (Aside: I've tried smushing and other png optimisations but the savings are not enough, a test of 2 folders in PS showed PNG 8 should be without significant degradation of the images)

When I tried PS CS3 Batch it wouldn't save over the originals and the new files it creates have no folder structure. Is there a way to fix this or another tool for the job?

I'm running OSX but have access to Windows XP/7.

XnView handles batch processing/conversion. Ctrl + U : "Tools -> Batch Processing..."

• Options to overwrite, use original path (as output), and/or keep subfolder structure.
• Add the "Convert > Convert to Colours" transformation from the Transformations tab. One of the parameters is bits/pixel.

That's a pain isn't it? Here's the trick. After recording your actions to make it png8, click the top right corner of the actions palette and choose insert menu item. Then just click file --> save. Click OK. It should now be the last sub-item within your action.

Now when you run the batch, stuff stays in its subfolder like it's supposed to.