Belt fed is still (for the moment) legal in California. Belts are considered the same as high capacity magazines, so it's illegal to link more than ten rounds together unless the links are pre-ban, or in the case of cloth belts, if the belt is pre-ban.
Similarly, it's illegal now to buy, sell or import high-capacity belts or pre-ban links into cali, so if you're not already belted or linked up, you've got no legal way to get some in cali.
The legality bit was that AR-15's are on the RR89 list, so can't be brought into Cali regardless of features, and must have been owned before the registration deadline, and registered to be legal for use there. If the FAB-10 is a 'california legal design' meaning it has been made not to take detachable magazines, then adding a shrike upper to it would probably not be allowed. Those with existing, registered "assault weapons" AR-15's would be able to mount the shrike on their lower, though.
Unless, of course, the ATF decides that the shrike upper is a firearm all by itself (like they did with brpguns' XMG-99 beltfed upper for the AR-15). Then, it would probably quickly be added to the RR list all by itself.