Anyone have a better workaround?
You make an in-place family while in workset 'A', then go back into that family while in workset 'B'. If you check its properties, it says it belongs to workset 'A'. If you go into edit-mode, some of it's parts say they belong to workset 'A' and some say 'B' depending on which workset was active when you created them.
Later, you have workset 'B' off, but can still see the entire in-place family. But, if you go to edit that family, the whole thing disappears (This is true whether you've got solid or void 'B' parts). To work on it, you need to make sure 'B' is on as well.
To correct this, make sure 'A' is the active workset and make sure both 'A' and 'B' worksets are on. Go into edit-mode. Copy/Move the 'B' parts out of the way by a specified amount of your choice. They are now 'A' parts, since the 'A' workset is active. Delete the original 'B' parts. Move the "clone" 'A' parts back to where their 'B' predecessors were. Close out.