You could modify the EDID, but most likely it must be hardware-compatible, meaning the aspect ratio will have to be respected.
The EDID contains the name and specifications of the monitor's technical data including it's capabilities (res, refresh).
The EDID can be re-written using special software.
Theres a python script called edid-rw that can be found at GitHub.
It reads / writes EDID data on the i2c channel between the PC and monitor.
The only way to see if you can actually use a different aspect ratio is to test by changing the EDID; but if it works the image will be distorted since one ratio will be altered to fit another.