How to control auto-generated grub entries for other operating systems?

On my computer I have Ubuntu 19.04 intalled along with Windows 10 and Fedora 29.

In Ubuntu, running sudo update-grub gives the following:

Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.0.0-13-generic
Found initrd image: /boot/initrd.img-5.0.0-13-generic
Found linux image: /boot/vmlinuz-4.18.0-18-generic
Found initrd image: /boot/initrd.img-4.18.0-18-generic
Found Windows Boot Manager on /dev/nvme0n1p1@/EFI/Microsoft/Boot/bootmgfw.efi
Found Fedora 29 (Workstation Edition) on /dev/mapper/fedora-root
Adding boot menu entry for EFI firmware configuration
done

which I think looks fine, it detects both Windows Boot Manager and Fedora 29 and it sets up a grub menu letting me choose between the different operating systems at boot time.

My problem is that Fedora needs some special configuration in order to workaround some graphics driver problems. Specifically, the options rd.driver.blacklist=nouveau modprobe.blacklist=nouveau need to be added for that grub menu entry.

I am able to fix this by manually editing the file /boot/grub/grub.cfg but that change is overwritten next time update-grub runs, so it is not a good solution. Also, the file /boot/grub/grub.cfg that I am then editing starts with a comment saying "DO NOT EDIT THIS FILE", also indicating that I should perhaps not edit it.

My question is: how can I add some extra boot options for the Fedora grub entry that is generated when I run sudo update-grub in Ubuntu?

There are some options in the /etc/default/grub file that is used by update-grub, for example I can choose GRUB_TIMEOUT there which works fine, but as far as I know that file does not allow me to specify boot options for other operating systems, only for Ubuntu itself.

To further clarify what I mean, the auto-generated /boot/grub/grub.cfg file (created by update-grub) contains a section looking like this, specifying how Fedora is booted if that grub menu option is chosen:

menuentry 'Fedora 29 (Workstation Edition) (on /dev/mapper/fedora-root)' --class fedora --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-6398dc3c-28e3-4947-ad82-d9defe5d9a28' {
    insmod part_gpt
    insmod ext2
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root  b00e70c6-820f-4615-b95d-c933eaea0d3f
    else
      search --no-floppy --fs-uuid --set=root b00e70c6-820f-4615-b95d-c933eaea0d3f
    fi
    linux /vmlinuz-5.0.9-200.fc29.x86_64 root=/dev/dm-2
    initrd /initramfs-5.0.9-200.fc29.x86_64.img
}

and I would like the above to look slightly different, adding the options rd.driver.blacklist=nouveau modprobe.blacklist=nouveau in the end of the line starting with linux. Is there some way to tell update-grub to do that?

I did see the suggestion here about editing the /etc/grub.d/40_custom file but that solution means writing the whole menuentry manually so it will not adapt when the kernel version in Fedora changes. The auto-generated menuentry created by update-grub has the advantage that it gives the latest Fedora kernel, I would like to use that and just add something to the "linux" line inside the auto-generated menuentry. Can that be done, and if so, how?