I have found my own answer to this!
All you need to do is as follows:
The additional attribute that you want to mass update on workorder object - add this to the object SKDWOMODIFYDETAIL - you can use same as (or the same as if that attribute already has a same as). Also create an addition YORN attribute with the same name and appended with YORN. You'll see similar attributes on the object.
Add both fields to the Modify Work Details dialog in Graphical Scheduler / Work Week.
And it works!!