Because so many people want this, I've made it.
Instructions:
1. Press Open, and select the save file to edit. Not guaranteed to work on any non US save.
2. Edit the trainer name to whatever you want. Note that any more than 8 unicode characters will be truncated without notice. (As in VashTheStampede being shortened to VashTheS.)
3. Press Save, and save the file
4. Important: Open Pokegen, load the file you just saved, select Pokemon Black2/White2 and English
5. Save the save file, without making any changes (unless you want to, but you don't have to).
If there are any other simple things (perhaps trainer id) that you may want to edit, then ask in this thread, and if there's enough interest, I may throw it into this simple program.
[Edit] btw, probably should have mentioned this requires the .Net Framework 2.0. Windows 8 doesn't come with it, so newer systems may need to search Microsoft's site for it.
Pokemon B2W2 Trai.zip