You've got BM, so there's a solution!
1. Load your PC into linux and install Gparted (if you don't already have it.
2. Load your into Backupmenu, Plug it in bia USB to your PC.
3. Hit "w" on the n900 for read/write usb mass storage mode.
4. Open up gparted. You should see all of the eMMC partitions - you can then make appropriate changes and be done.
5. Once that's done, flash the rootfs or load an optfs backup image via Backupmenu.
Your N900 should work again!