You can certainly try the approach I will outline below but I don't know if anyone has tried it successfully.
- Install virtualization software on your Ubuntu machine (VMWare, Xen, VirtualBox).
- Install Microsoft Windows Server in the virtual machine.
- Install MS SQL Server on the newly installed Windows Server.
I don't know if any other way would work but people can correct me the MS SQL Server from what I can remember actually relies on Licensing for Microsoft Windows Server. On top of this SQLServer is a pretty bad resource hog so normally organizations try to segregate it from running with any other applications on it's own cluster or server.
One thing that I would question is why not try Sybase as the backend? The connectivity from Linux to SQLServer and Sybase can go through
FreeTDS, which would look identical to your client software.