I just thought I'd give an update on this.
We have survived without a ribbon until now, but we are considering upgrading to Excel 2013. We have discovered an amusing(?) bug whereby running Excel 2013 with Bloomberg installed somehow deletes the "Add-Ins" menu. No solution to this yet... but I thought I'd revisit the ribbon idea as a potential workaround.
I did spend a while looking at the win32com idea, and, although I eventually got it to work, it was a bit messy because it does register an add-in which (unless you're careful) gets left behind in your Excel install. Also, it wasn't clear that I could get the pythom com server to work successfully without installing the components on each individual users' machine - which would be a bit of an overhead.
Instead, I added some xml to an xlam (which we already load), using the Office Custom UI Editor (http://openxmldeveloper.org/blog/b/openxmldeveloper/archive/2006/05/26/customuieditor.aspx
I then made the onAction event call xlRibbonOnAction, and set the button id to the name of an pyxll xl_macro function. A 3 line vba function in said xlam then on-calls to PyXLL via Application.Run.
Obviously, native support for the ribbon in PyXLL would still be best (we can't call PyXLL->Reload from the ribbon, for instance). But as a workaround, it's not too bad.