I did not read the referenced post but what I did when I did a remote session with you was:
1 - select all files for the album
2 - press alt + tab to open tagging in the action window
3 - entered the (Multiple Artists) with parenthesis in the [Album Artist] field
If the [Album Artist] field is not visible click the little wrench in the action window header and select also show [Album Artist]
Sorry I did things too quick, force of habit and it doesn't feel quick to me because I have done it so many times, I guess the hand really is quicker than the eye.
Edit: If you have multiple (Multiple Artist) albums like I do you may want to enter (Multiple Artists) [Album] depending on how you group your albums.
Replace [Album] with the actual album name no brackets. I use the brackets to show it is a field just like you do for an expression.