As you mentioned, caps could very well be the issue. But I'd be surprised if they're all that bad from POST/Bong and the HDD activity. At this point, I'd very highly recommend sourcing a video cable to localize the problem to the display if indeed that is your problem. Anyone with a video out capable 'Book or expecting to collect one should really have that cable adapter in their bag of tricks. You've been poking around in the 3400, so open it back up again to check for proper LCD cable connection and a possible tear in the ribbon cable.
Have you tried running it in SCSI Disk Mode from another Mac? If it shows up on the desktop that would indicate at least low level function. If you install a music file, name it "A" an put an alias for it the desktop items folder(?) you should be able to power it up, hit A and then return to play. Music to your ears would be just that, localizing the problem to the lid.
If it doesn't come up in SCSI Disk mode, I'd think you have a serious problem. SCSI Disk Mode cable should also be in 'Book owner's go bag. Including the wonderful PB100 NoteBook, every 'Book made between the awful first gen PB100 series and G3FW should have that feature, no?