I have found a solution to transfer music from a PowerPc to an iPhone 5 but it is a bit long winded and involves borrowing a modern Intel Mac. While this solution works, please be careful as other ways I tried ended up duplicating a lot of my music library, which I had to undo manually.
1. Back up your iPhone 5 to iCloud if you have already put stuff on it (just in case)
2. On the borrowed Intel Mac, set up a new account and make sure it is running iTunes 10.7 and use this account as it can be deleted later if required.
3. From the Intel Mac connect to the PowerPC and browse for the iTunes Music folder, which is in the iTunes folder inside the Music folder, where you will find the Artist folders, with Album folders inside them (list View is best here). You may have to enable sharing on some folders first, if you cannot get through, but I think it is unnecessary.
4. Open iTunes on the Intel Mac and, from the iTunes Music folder on the PowerPC, drag each of your Album folders onto the LIBRARY area at the top left of iTunes (I did it one by one but it may be possible to do several at a time). IMPORTANT - do not drag the Artist folders, just the Album folders with the tracks inside. iTunes on the Intel Mac should then have all your music in it, with the album covers and other data etc.
5. Plug the iPhone 5 into the Intel Mac and ensure that none of the Sync boxes are ticked in any of the tabs across the top. IMPORTANT - ensure that Manually Manage Music is ticked under Options in the Summary tab (and possibly Back up to iCloud).
6. Click on LIBRARY, Music and from the Albums window in iTunes, drag each Album onto the iPhone 5 icon in DEVICES on the left hand side of iTunes.
7. Finally eject the iPhone 5, give the Intel Mac back to its owner and enjoy your music.
8. If you want to transfer Playlists, first export them from the PowerPC (File>Library>Export Paylist) and then import them into iTunes on the Intel Mac and drag the playlist onto the iPhone 5.
Please note that while this worked for me for my old PowerBook G4 running OS X 10.5.8 and iTunes 10.6.3 using a borrowed MacBook Air, I cannot be responsible for any disaster that may befall you if you follow these instructions.
I hope this helps as I have searched high and low for a better solution.