I've only taken a very quick glance at it, but am I assuming correctly that the way packaging for SW works is that it's centered around .esps, and will include external resources referenced in new/modified records? So you can add meshes, textures, sounds etc, but no interface .swf's for example. Is that correct?

See Archive.exe in the root Skyrim folder. I think that's what your looking for.

Yes, I believe so.

That looks like a utility to create .bsa's to me, how is it related to SW?

Well, that's too bad. I was hoping I could upload SkyUI to SW.
I expected that the SKSE plugin would probably not be acceptable, but we could've gotten rid of that eventually by having everything integrated into SKSE itself (which we already did 99% anyway), so it would've really just been .swf's and text files.

When a mod is published to the Workshop, a .bsa alongside the .esp is uploaded, containing any extra files the mod needs. (rather than dump loose files to the game directory)

Ah ok, after some more experimenting I noticed that you can drag&drop files to the archive manually *cough*, and it accepts .swf's and text files. So it's really just the SKSE plugin that will require some changes.

Glad to hear it

When you go to upload a mod is also asks somewhere along the line if you need to include any additional files.