I'm happy to answer any questions folks have around this product. We've released all of the main specifications at this point, along with documentation and CAD on GitHub for the new module systems.
I think I remember seeing in an LTT video, mention of possibly using the expansion slot as an additional battery input. I didn't know if that was a possible direction considering the tech specs mainly focus on the 8x interface. Having bought a thinkpad for that exact purpose of an extra swappable battery, that would be a big boon if it was possible (even if you can't comment on a specific road map).
We've designed the Expansion Bay module system to be able to handle power bi-directionally. One of the main reasons for this was to enable secondary batteries. This specific module isn't something we've shared any plans around, but it's something that either we or a third party would be able to build.