This has been the only report so far of a system freeze with new Poseidon. You could try to use the old mass storage class from V3.4 to see if it really is due to the new massstorage.class, or if the problem is somewhere else.
Does this freeze also happen with no cards inserted into the cardreader?
About the AVR Battery Backup, I'd be very interested to see a PsdDevlister output of it and, a HID debug (there's a menu item for it), if it really uses the HID.class.
