EvilGenius

Update — Working fix available
I've got eMMC detection working on my Radxa Zero 3W with Armbian Trixie (vendor kernel 6.1.115, SD card boot). The fix uses a device tree overlay to inject the missing voltage regulators, pinctrl references, and eMMC capabilities into the base DTB, combined with a small systemd service that rebinds the SDHCI driver to work around a deferred probe timing issue.
After applying, the eMMC is detected as expected:
mmc0: new HS200 MMC card at address 0001 mmcblk0: mmc0:0001 8GTF4R 7.28 GiB armbian-install works normally after that to migrate to eMMC boot. The rebind service is conditionally skipped when booting from eMMC so it doesn't interfere.
 
I've attached a zip with all the files.
The README.md inside has the full details — root cause analysis, what was tried and failed (including full DTB recompile), how the fix works, installation/verification/revert steps, and known limitations (the pinctrl phandle values are specific to this kernel build).

I still working on a version that does not need the rebind service for sd cards. but for now you need the rebind on the sd card to get it to find the EMMC so you can install.
emmc-fix.zip