I'd say do a binwalk this file first. Then see if there's a comparable openwrt image to use.
Did you check this one out https://forum.openwrt.org/t/openwrt-on-old-cisco-asa-5525-x-appliance/127704
As @ppmm said check the format of the image. I assume it's probably some kind of elf executable. Since this an old appliance, the bootloader might not be capable of booting too big images. Maybe preparation of second stage loader (grub2 or U-Boot) might be needed. There are some information on ROMON for mips, but it might be useful in Your case:
.https://github.com/pvachon/cilo
https://www.linux-mips.org/wiki/Cisco
Due to the fact that I don't want and have the time to invest into this project (judging now it would be a lot of time) I will stop investigating further, maybe I will come back in the future. But thanks to everybody who helped in this thread!
If it's any consolation, a Geode processor is dog slow (though also very low power). A MT7621 will run circles around it.
I would try booting the initramfs image, that can't do much practical but it could at least show OpenWrt booting. An intramfs is everything ready to run in a single bin file.
Where do i find such an Image?