I tried it and it is possible!
In the script safe_save_use_aes I put the key as the third argument and I check there if the key is an array. If yes, then AES functions are returned otherwise -1.
For this in save_save.gml I had to modify the calls of safe_save_use_aes to provide the key as third argument.
:-)
It seems to work without any problems! Anything I might have missed?
Thanks!