<CustomTag>.displayFormat

[Gdg]

I want to see it in the main browsing list.
Thank you, Simon, that works, but... with a side affect.
All the albums without the BoxSet tag, are grouped under their "Genre" tag.
Just because a BoxSet tag is virtually generate, even for tracks that don't have it.
Is there a way to avoid this?
The requirement is:
If a track has the BoxSet tag, then append the Genre tag, else do nothing.
Thank you