I finally got around to booting the laptop with NetBSD. I used the booter application and a net install .gz image file. The kernel reports the FPU is being emulated. I'm sure if this is due to booting from inside MacOS or not. I've tried getting it to boot natively from the hard drive but it seems my power adapter is going bad - it is cutting power to the machine randomly.
I think it's definitive - you can't upgrade a PowerBook 500-series to a full FPU CPU and have it work properly.