Although I've converted the first 8 zoom levels of our map tiles, the final 3 zoom levels contain 5,505,024 of the 5,680,042 images.
I've adjusted the script a bit to include a nice-ing of the job. Tests suggest that each image will take around 4.25 seconds to be converted. This is rough, but a guesstimate of the total time to convert the remaining images is about 271 days. I do it one zoom level at a time, then. Zoom level 9 contains 262,144 images, and should take about 17 hours. Level 9 is also the last zoom level with decent imagery. At 10 and 11 the images are very pixelated and not really worth the effort or time.
So, I'll set the script to converting zoom level 9 today.

UPDATE: Somehow, the ImageMagick version on arugula got rolled back, so my script (which relies on hald-clut being available), failed; so I need to do this locally for now. Currently moving z9 (262,144 files) to my local machine for processing.