As noted in this thread about emulating Target Disk mode using a Linux computer:
    I discovered my test mule Aluminum G4 PB had a borked internal hard drive. (And a borked memory slot, but I'm ignoring that for now.) While browsing everyone's favorite e-Commerce big brother for the right FireWire cable to do some additional testing on another machine I stumbled across a listing for a laptop-size PATA to M.2 SATA adapter for $13. (The particular seller for the one I bought is sold out now. Here's the same one listed for $20, but someone else may well have it cheaper. So, just for laughs I bought it and The second-cheapest 128GB M.2 SATA stick that didn't have terrible reviews to see if it would work as an HD replacement. The bits got here today:

And so after the expected hassle I got it installed in the Powerbook. (Side note: Jeebus Xrist, I'd apparently blocked out just how terrible it is to replace the HD in an Al PowerBook G4 despite doing it fairly regularly once upon a time. That stupid little yellow cable is basically folded sharply in two when the HD is fitted and that makes it almost impossible to get the little bugger plugged back into the motherboard... aaaargh!)

TL;DR, it seems to work fine. I booted the laptop over Firewire from my Linux computer as described in the linked thread, installed OS X 10.4.6, and let Software update do the combo update. It boots in about 20 seconds from the initial gray Apple logo to the Desktop, and TenFourFox is actually close to usable on the system, which is particularly amazing considering at the moment I'm stuck with only 512MB of RAM. It's quite remarkable how much it helps the computer, well, suck a lot less.

I stumbled across this thread of people posting Xbench results for various SSD-in-a-PowerPC solutions; this particular combination of parts nets a disk score of 218.94. This seems to be near the ceiling that people can get with this laptop if the results of that thread are accurate. Total BOM was about $40, so for the price this seems pretty darn good. (I don't want to slag on OWC's turnkey solution, but there's a lot of difference between $40 and $130.)

It looks like mSATA-IDE adapters with a similar laptop form factor are even cheaper so maybe I should have gone with that, although the drives are slightly more expensive so I guess in the end I would have broken about even. In part I bought a 128GB drive instead of a larger one because I was kind of curious if the drive would be compatible with my B&W (assuming it still works) when fitted with a 40-to-44 pin desktop adapter, but after what a pain it was to get it into the PowerBook I don't think I feel like pulling it back out. Anyone tried something like this in an older machine?

Also wondering how durable this is likely to be, but I'm guessing I probably won't use the machine enough to stress it too hard.