It shouldn't be too hard.
Both options are relatively easy;
Option 1:
Use a hot air rework station to remove the flash memory. Then clean it up and re-ball it. Attache it to a appropriate flash programmer. [These can be super cheap MCU contolled bit-bangers or multi-thousand dollar professional equipment. With most of the more affordable ones landing in the several hundred dollar range.] Read and backup the chip.
Re-root the file system in memory and then write the modified file system back to the flash chip.
Depending on the flash programmer, re-ball the chip again if you had to solder it to your programmer. If you have a nicer setup with the BGA suspension sockets you wont need to re-ball again, which is nice
Then solder the flash chip back to the board. Finger crossed, you'll be re-rooted.
OR
Option 2:
Use the same hot air rework station and remove the CPU. Attache very thin wires to the uart directly with a level converter if not an optocoupler. I don't know what pins but if you're interested I can lookup the chip and tell you what pins are likely. Attache the rest of the pads back to the board with very thin wire.
[I think this illustration will help.]
BOOM, serial port on your Oasis!
Root the Oasis via serial. Once your done re-ball the CPU, pop it on the board with a liberal amount of flux and hot air.
You're done!