O.K I can confirm that you can transfer PAL save files to NTSC-U.
All you need is Savegame Manager
here and a hex editor (I use Hex Editor Neo as it is free
here).
1. First move your NTSC-U save to the SD (for backup purposes). It's the one that says "A new Mario adventure!"
2. Run NSMBW and create a new NTSC-U save without starting the game (you can exit the game after the "creating new game save" message).
3. Open Savegame Manager and select "extract" and "SD".
4. Find your NSMBW saves (should be listed twice, one for each region) and extract them.
5. Now take your SD card and put it in your computer.
6. In the root of your SD card you will find the "savegames" folder. Make a backup of it just in case.
7. Open the "savegames" folder and you will find two more folders. The one that ends with "45" is NTSC-U and the one that ends with "50" is your PAL save. Open the PAL folder (00010000534d4e50) and open "wiimj2d.sav" with your hex editor.
8. The very first thing you should see on the text side is "SMNP". Change the "P" to "E" (yes it is case sensitive) and save.
9. Copy the edited "wiimj2d.sav" to your NTSC-U folder (00010000534d4e45), and delete the PAL folder.
10. Now insert the SD card back into your Wii and launch Savegame Manager.
11. This time select "install" then "SD" and install your NSMBW save.
12. You can delete the PAL save file after you confirm it works. The PAL one says "Play Mario with your friends!," and as stated above the NTSC-U says "A new Mario adventure!"
Now you should be where you were in your PAL game. This works for both USB and disc loading. Hope it helps!