Unfortunately it's impossible because the fpga core file is a compiled binary file (including crc). Even with included rom image you will not find any of C64 rom fingerprint inside "minimig1.bin". Its all extremely translated to run inside the fpga as block-ram and logic gate components.
I may be able to store the rom image in s-ram by using an Amiga program. This would allow fpga64 to handle different C64 rom image and the fpga is not that loaded. It also would need to start the Amiga core first, upload the rom image to a special ram address and reboot Minimig with a different sd-card to use fpga64 core file. To make this work I have to reorganize those fpga64 rom module structure and to write an Amiga assembly program to pre-upload the rom image files.
Its all a bit complex to use but maybe the only way to release the core file. Pls give me time