Investigate lthor / thor and samsung u-boot for Tizen
We already know of a method by which we can replace s-boot and replace it with u-boot, but it requires opening up the device and shorting a touch point on the logic board.
We have been looking for a method by which we can replace s-boot via software only techniques. One such technique might spring from the work being done here: https://github.com/oranav/i9300_emmc_toolbox
Another option, if heimdall by itself doesn't prove in the end to be enough, might be lthor.
This upstream u-boot ODROID exynos4412 repo shows that lthor/thor is enabled: https://gitlab.denx.de/u-boot/u-boot/blob/master/configs/odroid_defconfig
It was suggested to Wolfgang in the last email of this thread that he should use lthor to "convert" an i9300 to an RD-PQ/Trats2 board to get u-boot on the device: https://patchwork.kernel.org/patch/9345815/
lthor is free software, but the i9300 doesn't have lthor support by default.
On post #77 here https://forum.xda-developers.com/showthread.php?t=2482563&page=8 it is claimed that a modified kernel with access to the mmcblk0boot0 partition was used to flash a modified version of s-boot, which was dumped from a RD-PQ, that included u-boot with lthor enabled in it. Much of the same information is reiterated by the same author in this post: https://forum.xda-developers.com/showthread.php?p=64794497#post64794497
It is my understanding that once you can use lthor with your device, you can then very easily flash any image that you'd like onto the emmc using the free software program lthor. At least that is the suggestion after reading to the end of this section of a Tizen wiki: https://wiki.tizen.org/Exynos#Creating_the_image_manually
https://wiki.tizen.org/images/b/bd/Lecture-1.pdf has some information about creating updated versions of u-boot-mmc.bin, which might come in handy.