This is probably impractical, but if you have additional storage on the machine, you could always set up another small installation on the side and boot to it.

It seems like the flash drive with the issue is the disk / is on, so it may be more complicated than this, but short of a LiveCD, and with no access, that might be your best shot.

If you try it, make sure you make an additional GRUB entry instead of overwriting the original in case the new one doesn't boot correctly or has network issues (because of the SSH.) It seems obvious, but my idiot self in a similar situation with SSH access only made it impossible to connect through SSH to my system and I had to go directly to the machine.

Hope this helps.