Agree with all comments.
Someone really need to come out with a multi-channel P-channel MOSFET array chip for sourcing current.
The biggest I've seen is 4 channel
and the Rds is awful, 180 ohm.
I think I'd go with discrete p-channel mosfets
this one seems a good compromise of pretty low Rds, low cost, for a standard part (12V, use an open collector/open drain driver in front of it to turn it on, or a shift register like TPIC6x595 (6A, 6B, 6C, 6D from avnet.com, compare prices))
Or this one
if you're sourcing current from 5V and and control it directly from ATmega outputs, of shift register like 74HC595.