>Which can probably be mitigated by having a battery underneath the keyboard as well, to be hooked up as a power bank, but the setup won't be very elegant.
I reckon it could be done... The Moto Z actually has a fairly small internal battery, and a two way power/data magnetic connector. If you place a second battery under the keyboard module, and use a cunning sliding mechanism to place some support (a la the original Psion) it should be stable.
It isn't necessary that it work with every phone, just each year's new Moto Z equivalent.
[By the way, I would like to thank you all for your understanding; I've harped on quite a bit about Moto Mods this last two weeks, and you've all understood my enthusiasm has been for the possibilities such a system could open up - especially if open sourced or licensed out -and that I haven't been cheerleading for Motorola per se. In fact, I feel a bit sad that it is a proprietary connector]