It seems that this is not possible, if this answer is correct, saying:
You most likely have Solus installed while the bios was set to UEFI
mode. This will install Solus with the clr-boot-manager bootloader and
not grub. Hence there are no grub entries created and hence Mint Grub
will not have the entries available to boot. Reinstall Solus while the
BIOS is set to 'BIOS Legacy' to install Solus with grub boot entries.
The fact that the entry is located in loader/entries and the file
extension is conf shows that it is not a grub entry. Grub uses .csv
files in the same folder as the .efi.
...
... Consider using the BIOS itself to choose an OS to load (and the bootloader for which version,
LTS or current, to boot) instead of one single bootloader for
everything. In my experience that will save a lot of trouble when
messing around.
From later experience I can say that, after restoring Ubuntu grub, this was able to see other Linux systems that don't use grub either (e.g. KaOS, which uses systemd-grub; installed after Ubuntu, it imposed its own boot menu which was lacking an Ubuntu entry, thus hiding the Ubuntu grub list under Firmware Interface/BIOS settings).
I'll mark this as this answer for Solus, at least for now.
