My HP Passport registration was sufficient to open KM438391. However, there are still no news there regarding Open UI support. I personally have not (yet) come across any other tool claiming the specific support for this particular Siebel GUI variation. I believe you can as well use any other good Web testing tool to practically achieve the same (bad) results. They all see the GUI as "plain" web objects and provide as much standard support as they can. You will have to solve plenty of problems regarding object identification and you will have to handcraft any other solution for "special" objects. It is by far not impossible to test Siebl Open GUI. But it is rather painful and hard to do. And you cannot simply convert your old tests to work with it. At least not to my knowledge.
Just a short addition to Anna's comment regarding "basic Web" add in.
We have discovered that if the "good old" Siebel AddIn is enabled (instead of basic Web Addin) on Siebel Open UI, then important Siebel attributes such as "reporitoryname" get exposed as HTML attributes for most of Siebel's simple objects like edit fields, check boxes, etc. Complex Siebel widgets such as list applets as still problematic and require lots of ugly hand-crafted code but the "simple" part works fine which is quite a lot.