… several hack-compile-fail-hack iterations later …
Well, I had managed to get more-or-less working image with in-kernel ATA support, working jffs2 overlay and full 2GB of memory available for use. Hacks I’ve done to make it happen are quick-n-dirty changes to DTS (proper block2mtd config for mtd emulation over SD card; fix to enable full 2GB of memory), kernel config (enable AHCI_MTK, ATA and SCSI), a fix to vendor-supplied hack that builds eMMC and SD images (do not require/use initramfs), some misc fixes to openwrt’s mediatek-specific makefiles. In current state changes are too dirty and hackish to share. My plan is to tidy them up and create a PR to vendor’s repo (and - with some luck - also to upstream). Although don’t make hopes too high as my free time to spend on this project is really limited due to Real Life ™ so ETA would be more like weeks than days.