As i guessed in a few days ago that lede support would be coming soon and it did happen. is there default uboot on the emmc boot? that what i meant is maybe you can use to upgrade the image through the default uboot in emmc with the result you built "lede-mediatek-32-mediatek,mt7623-rfb-emmc-sysupgrade.tar".
Othersiwse, you have to apply it through sd booting which i felt that is
hard enough to general people in especial not familar embededd linux developement. but i also address them in the rough sequences as below ...
1) firstly, build a sd card with able booting
2) secondly, using the uboot from step 1. to replace the old uImage with lede-mediatek-32-mediatek,mt7623-rfb-emmc-sysupgrade.tar, which i guessed it should be the certian ramdisk helping to upgrade with the normal and big image writing down into emmc.
3) thirdly, boot to shell with new image applied from step 2.
execute "some command" to upgrade normal and big image i said in step 2.