mSATA SSD via IDE-adapter in PowerBook G4 12” I have a beautiful PowerBook G4 12” 1 GHz PowerPC ((A1010 (EMC 1986)) here with a dead hard drive which I want to replace with a SSD.

Is there any reason why a mSATA SSD (with mSATA to IDE adapter) would not work on this device?
Those adapters should work in principle for PowerBooks of that glorious PowerPC era. 

Although you might want to doublecheck your exact model.

Apple PowerBook G4 1.0 12" (DVI - Al) Specs
Identifiers: 12-Inch DVI - M9007LL/A* - PowerBook6,2 - A1010 - 1986

Apple PowerBook G4 1.33 12" (Al) Specs
Identifiers: 12-Inch 1.33 GHz - M9183LL/A* - PowerBook6,4 - A1010 - 1986

Initially I thought those models would still be limited to 120GB supported capacity, but some vendors offer up to 480GB for a model of that range.

You might also want to check whether it fits into your machine. But that shouldn't be a real problem. This one is reported as being used just the way you intend to: in a PowerBook G4, but sitting a little tight


  adaptare 46235 mSATA-SSD 2,5 Zoll IDE-Controller 44-Pin Adapter
  


Or as experienced and friendly commentator John Keates says:


  I can confirm those adapters work. Also used a 1.8 to 2.5 adapter, a CF adapter (but that is easy) and SD adapter. Also one m2 adapter but only the sata variant; have not seen a NVMe to IDE adapter yet. 
  
  Most issues were with unstable power for the adapter (resets during bootup) if it didn't have good DC-DC circuits on-board. There were a few where the adapter chip wouldn't allocate beyond 137GB, this is mostly because they completely emulate a disk and fake the on-disk format (but do pipe the data to the underlying disk just fine). There is an issue with some chipsets that don't do the right protocol switching on the IDE side, like going through PIO and DMA modes, but I haven't seen that type of issue for years. Note: last time I used such an adapter was about 4 years ago on OSX10.3 (iBook G4)