This is fundamentally possible as the user guide states: "The internal flash memory is programmable via JTAG interface (in-system) or directly with user application code (in-application)."
However the issues arise if the SLA covers the entire flash area. Even if the program itself does not cover the entire flash area, the bootloader script will write the cryptographic hash to fill the entire flash.
The solution to this would be to change the bootloader script so that it does not include the entire flash area including the cryptographic hash.