You might be interested in the "treat Album Artist as" setting in Preferences > Sorting/Grouping. With that ticked, if you leave the Album Artist blank for your Various Artists albums AND tick the iTunes Compilation flag on the settings page of the Tag Editor, it will assume whatever Album Artist you define in Preferences.
Thanks for the suggestion. That's interesting, correct me if I'm wrong, but I don't think it is feasible in my use case with over 1,000 albums that are compilations with "Various Artists" as Album Artists (with performing Artists as <Artist>) for use in different media players. More importantly, I have more than a few autoplaylists on those players that rely on <Album Artist> = "Various Artists". Anyways, based on your suggestion, MB would seem to treat blank <Album Artist> fields as whatever I put down in the Sorting and Group submenu. It suggests "Various Artists," which brings me back to the beginning of my question. How to custom sort Various Artists using a sorting subrule embedded in the sorting role.
In other words, I'm hoping to create a virtual tag for sorting in either of the following way:
IF <Album Artist> = Various Artists, then sort these albums via <Album> and then <Track #>, ELSE sort by <Album Artist>, then <Date>, <Custom Date>, <Album>, and finally <Track #>
IF <Album Artist> != Various Artists, then sort these albums using <Album Artist>, then <Date>, <Custom Date>, <Album>, and finally <Track #>, ELSE sort by <Album Artist> and then <Album>
EDIT: Temporary solution (for me) is the following, use the code below as a virtual tag:
SortVA = $If(<Album Artist>="Various Artists",$Group($Sort(<Album>),1),$Group($Sort(<AlbumArtist>),1))
Then set up custom sorting with the following in order: <Album Artist> / <SortVA> / <Path> / <Track #>
I had to use <Path> because there were only up to 5 different sort orders. With six, I would have done: <Album Artist> / <SortVA> / <Year> / <Custom Date> / <Track #>
My custom date is a custom field consisting of <month.day>. This is a hold over from mediamonkey where I would rename my directories as <Genre>\<Artist>\<Year>.<Custom Date> <Album>\etc, etc... Thankfully, my OCD allowed <Path> to work fine in this instance.