There seems to be very little hope if it wont power on using battery or power adapter. I guess you could try a replacement power adapter, but it sounds like it wont make much difference. The first thing I would put the problem down to is the DC In board. It more than likely to be that.
If I'm being on honest here, it's an old laptop and unless you have known it to be working before, then it could be anything that is wrong with it. You could spend an awful lot of time diagnosing the problem and find that the logic board has gone on it.
There may be a common fault with these PowerBook that I'm not aware of and someone else on here may know about.
Best of luck
Here is the flow sheet from the Apple Pismo manual.
Computer won’t power up
1. Restart computer by pressing the reset button on the back of the unit. Wait five seconds and press the Power key.
2. Try known-good power adapter.
3. Try known-good, firmly seated, charged battery. Repeat.
4. Connect power adapter and restart computer in 3-4 minutes.
5. Disconnect internal keyboard completely and try powering up unit. If unit powers up, replace keyboard.
6. Verify microprocessor card is firmly seated.
7. Replace sound card.
8. Replace power supply card.
9. Replace microprocessor board.
10. Replace I/O logic board.
Make sure the cpu is pressed firmly on to the underlying socket board see #6. I had this problem.