The current draw (as you said, averaged over a 5.12s time span) can be read from the BQ27200 chip. You can either use the bq27x00_battery driver (in kernel-power) and read the value from /sys/class/power_supply/bq27200-0/current_now, or talk directly to the BQ27200 over I2C, using i2c-tools. Read the relevant register with the command

i2cget -y 2 0x55 20 w

In both methods, note that the units are unknown, so you can tell when the N900 uses a lot of power or a little power, and you can tell the ratio between a lot and a little, but you can't know exactly how much mAh are drawn.