currently the diameter of a carbon clutch is 4inches...the drawing of an AP clutch states 138mm diameter and around 50mm in depth .As the bottom of the Gearbox/bell housing is not round there is definitely some space in this area in the bottom area,but admitedly the picture I found does indeed look like the bellhousing is tapering severly in the bottom area towards the back .
Considering the guesswork included when you have to decide on things like weight distribution AND building a new gearbox for 2010 it seems fair enough someone at least thought of making allowances to put ballast somewhere on the underside of the box .As it was stated more than once ,Brawn had no chance to test the new tyres and had (they did have Michelins before,did they?) no experience with the old Bridgestone rear tyre wich is reintroduced for this year so basically they ended up with something like Virgin did with their fueltank ,obviously a misjudgement on a different level but a misjudgement .
I would feel quite bad if I had supported or decided on this whatever the reason were and now having to redo something as elaborate as a new gearboxhousing.
http://www.apracing.com/drawings/cp8153cd.pdf