Not particularly interesting, but if you're at all curious about what's accepted for PCB manufacture in terms of multiple boards separated by anything other than silkscreen (it's not possible to get a straight answer from anyone - I tried!) then this had no problems whatsoever:
Although the other bits don't really count as "boards", and of course YMMV.
I also screwed up (finally)! I completely missed a connection to VREG_IN, so it's got a microscopic bodge wire:
The fake micro sd cards worked well - 0.8mm being the upper limit from the specs so might be tight in some sockets without sanding it down. One unavoidable problem is there is 1 corner that a 0.8mm milling bit cannot quite cut far enough (because it's round...) but it doesn't seem to make any actual difference in use, and nothing that couldn't be fixed with a small file.
Allwinner Axx SoCs have JTAG and UART mux'ed on the SDIO pins, so these adapters are meant for debugging "China-pads". The dummy card and eMCP test were really just to fill space :) I would not recommend trying to get a 0.5mm pitch BGA using NSMD pads on a cheap PCB - it was that badly aligned it wasn't usable. Having said that you also wouldn't have space to route more than a few traces anyway... 0.8mm isn't possible if you intend it to pass a DRC :(