I don't know XCSoar, I guess you'd be better off discussing it in the XCSoar forums where people already have successfully done this for the Aura?
The Aura [lack of internal SD card] is not a good platform for modifications in general, and particularly dangerous for those that modify the boot loader / very early boot process itself. Basically you have one shot: if it works, great, if it bricks, game over. Unless someone finds a way to re-write the onboard flash by some other means even after everything else stopped working. Serial? JTAG? ...
Not sure if anyone has ever managed to recover a bricked Kobo Aura, the only JTAG related thing I found was something slightly different
http://nahitafu.cocolog-nifty.com/na...jtag-5cd0.html http://nahitafu.cocolog-nifty.com/na...jtag-c64d.html
Would XCSoar run in the Linux environment provided by Kobo? If you could let Kobo boot into nickel first and only then kick-start your own software, it might be more forgiving to errors...