@shwishblade Well your method works! I just had to install the pixel desktop for retropie from the retropie_setup.sh script (I didn't even know we could install the pixel desktop... neat!). It's a shame indeed that there is no way to run the game outside of play_swing. I also noticed that the game has trouble playing some sounds.
A step in the right direction, let's hope it will be possible in the future!