I see.

9front's booloader works flawlessly. It's a little annoying to write
local!/shr/sdU7.0/tmp/9front.iso at the command prompt everytime, but at
least it runs. The only problem (aside of using the US QWERTY layout, I'm
spanish) is that it cannot run rio (it says something like 'cannot open
/dev/draw/new: no frame buffer', I will investigate why it is happening).
But now I can use 9front, and I'm happy with that. Now I will try running
Plan 9 with 9front's FAT-based bootloader and see what happens.

Thank you for all the help, guys.