A couple of years back on a trip to Japan I bought a Sharp NetWalker PC-Z1. It came with some old version of Ubuntu and I had planned to put Arch on it at some point. However at the moment it is just sitting, collecting dust.

Lately I had thought about digging it out and trying to get Slackware ARM running. Before I do wonder if anyone else has tried this and can comment on the likelihood of success and/or point out any tips or suggestions. If not, I guess I'll just have to dig it out and give it a go.

I did search the mailing list (keyword: netwalker) but it turned up nothing.

Well, if it runs the ARM variant of Ubuntu it almost certainly could run Slackware too. Installing it is the trick.

Looking at these resources online, it seems your biggest challenges would be building/updating the kernel for it, and getting the bootloader to start up a Slack image.

Debian installation info
Kernel compile and flash

Though it looks like you can run the OS off of SD, which should be safer and easier.