I have read that MWM had WSM in the past, and that is included in the demos/programs folder in the source code for 2.3.8.
However, there is a note stating that the functionality to support was removed in 2.1.
As Motif is quite a legacy toolkit now the information available on this is obviously limited, I am wondering if somebody would still now (or remember from past days) which are the options to implement that kind of functionality in Motif.
I know such functionality is included in CDE which is now open sourced, but I do not like CDE.
There're four Mwm versions available in addition to the original one:
The version included with LessTif, which has support for virtual desktops built-in. I'm not sure about WSM API compatibility, though.
There's a fork of Mwm called EMWM which is actively maintained. If any functionality you need is missing, I believe patches will be welcome.
You can build CDE and use just the Dtwm window manager. It is basically Mwm 2.0/2.1 with WSM functionality retained.
Finally, there's a MaXX Interactive Desktop (source code) which contains a 5Dwm window manager and aims to provide the look-and-feel similar to that of IRIX Interactive Desktop.