I remember there was talk of an unofficial UI SDK a while back, and I just noticed a https://github.com/Mardoxx/skyrimui for it on Github. Is this the right project, and is it ready for use, or should we wait for a more official announcement from the folks working on it?

Your link appears to be broken, sport.

So it would seem. Fixed. Thank you, kind sir.

Well it's not exactly news We only recently moved to GitHub, but the files have been available since last year.
It is ready for use, but depending on the menu you're attempting to change, you might be the first person to do so. Don't be surpised if you run into some bugs. If you do, just report them and Mardoxx will most likely fix the issue.
I know that people have been using the dialogue menu in a mod, so that's tested. The HUD should be well tested as it's used by a couple of other mods.
The quest/journal menu is tested, too, because I'm currently working on that.

Stealing my thunder

HUDMenu is pretty much completely done now, that is the most difficult one to get working from a fresh decompile so I spent most time on that.

It's only me working on it so progress is slow and irregular (also I'm working on some cool stuff for SkyUI). If there are any menus you want me to prioritise give me a shout and I'll get them in a working condition. Most are, but the code is a bit of a mess from the decompilation.

Are we able to create new menus and such, or is that still a ways off/not possible?

Not very easily, no, but it is possible in some respects.
What type of menu were you thinking?
We're (schlangster is) working on a mod config menu for SkyUI which other mods will be able to register to, so maybe this will will be enough?