It worked !! I have a functional Rockboxed iPod Classic !
Well it was not the case at first, I had the same problem as AmberOzl :
When I push select and menu button for about 10 seconds to put the iPod in DFU mode (Apple logo comes and goes then screen stays black, then I remove my fingers from the buttons), my computer recognizes something and try to install drivers automatically but drivers can't be installed successfully. At the end, I don't see the new device in my computer as Apple Recovery (DFU) USB Driver, basically I don't see anything. So I can't continue.
Also, iTunes is not installed on my PC (I used the "drivers only" method), and AppleMobileDeviceService.exe and iTunesHelper.exe were nowhere to be found, they never launched. I checked "Show processes from all users" and no joy either. I found AppleMobileDeviceService in the Services tab of the Task Manager, but it was already disabled.
I tried to run the bootstrap anyway and it said it didn't find any device, so after looking around and switching modes a few times (holding select & menu for 12 seconds) back & forth, and still nothing in Windows Explorer (I'm on W7 Home Premium 64bits), I launched bootstrap again and this time it worked (no idea why...) I was finally able to see the 64M UMSBoot and proceed with emCORE installation and then Rockbox.
It's now working perfectly with the latest daily build, and when I unplug/plug it I can see the iPodClassic in Win Explorer. Now I'm off to fiddling with settings, themes and music, thanks !!