I want to copy the value of %artist% to %album artist% if empty. is this Possible?

not currently, although there is actually 2 $album_artist fields currently, one that is the content of the album_artist tag, the other fallback to the artist value when the album_artist tag is empty.
Some parts of gmb only show one of the 2 fields, SongList/SongTree let you add both, though they currently have the same name, which is a bit confusing.
Why do you want to do that exactly ?

I have a similar question and do not mean to hijack... :o

All of my tracks have incorrect albums based on an old style of Genre based structure I used to have..

I want to do a mass change of the Album and make it the Artist, is this possible with the tag editor, if not, would you know of a tool or script?

Tag editor Ex Falso allows editing not only tag value, but also the name, so you could select all your tracks and change tag "album" into "artist".

It's not possible currently, but I'll add this at some point it's rather easy to do. The harder part is finding a place in the gui to activate this, maybe an option in the mass-tagging dialog, though adding a way to preview the changes would be nice.
Also currently it would be limited to swapping a value by another, which is something you rarely want to do I think. But it will be more interesting once I add the "advanced syntax" where you can modify values based on some simple instructions (like I did in the SongTree, but limiting that to the SongTree was a mistake).

I appreciate the reply and the upcoming changes...

I ended up using puddletag which has the ability to swap a value for another, and have now replaced easyTag with this app due to it's speed!

Is it noted here how to keep GMB current with a git or an svn process?

Thanks!