There should be a command to turn XREF's on or off for the whole project.
At the moment, all I can find is a command that allows you to select which XREF's to hide. Nothing to hide ALL of them in the project.
Something like this should be available within the Project Wide Utilities.
There are two options for how this command could work.
Option 1 - Turn the specific layers on/off. This could be used for more than the XREF's so various options may be available here.
For the XREF option, I'd suggest a check box to select the XREF's and then the command should check each dwgs Layers setting for the layer/s assigned to the XREF's before turning them off. Same for other layers defined in that dialog. You could also select layers from a list in the current active dwg.
Option 2 - Go through each dwg and hide the XREF attribute.
I'd imagine this would take a bit of time to process each dwg depending on how many blocks there are to process.
Going with this option would not be as flexible as going with the other option.
Regards Brad