Is it "just" an internal USB-C connection to the expansion card? Or is there something else going on that could be more difficult to work with?

Nothing that goes outside the usb/displayport standards. We have to be compatible to support the passthrough card :).

This is a great first step. Ideally the expansion story converges on some kind of industry-wide standard, like PCMCIA was. Would be cool to have other peripheral manufacturers out there agreeing on the spec and committed to building expansion hardware!

Sounds like a good solution to me, I hope you succeed, I surely will keep an eye on the project!

I assumed it wouldn't be an open design because the marketing didn't mention it. I would integrate this message in your marketing because it's worth convincing critical audiences.

Is there a catching/locking mechanism for the expansion module that holds it in place? It would be a shame if the whole module came out when trying to remove a particularly firm USB connector.

Yes (source: another comment that described it. lock & button to remove. hot swapping supported)