Just had a roll bar installed and was told that I need to foam wrap it for open track competition.
How much of the bar gets wrapped and ware? Only the Diagonal section?

 

If you're going to be racing in sanctioned events, then you need to consult the GCR for your sanctioning body to see what type of foam is permitted and where it's required.

In general, SFI rated foam is required at any spot that your helmet might make contact with the cage in the event of a crash. YMMV.