So I used something like these some years ago to recover data off a phone, but I was wondering if the reverse is possible in having a bga soldered adapter with a microsd slot on top. Or if PCBs can even be soldered together like that. I've never actually checked if bga chips have raised pads or something. The purpose would be for rapidly testing custom firmware for shitty old devices that were designed to be replaced without removing the emmc to flash it separately.
If there are pads that break out the pins to the eMMC, then manually soldering very tiny wires is usually doable with steady hands. If test pads, then a jig/PCB with pogo pins can work.
For really fine pads, then it'll be quite the uphill battle, bordering on whether to even attempt it. I personally miss the days when devices had a MicroDrive or ConpactFlash card hidden inside.