--Every 100 ms or so, wake up (via a Timer2 interrupt)
Were you planning on attaching an external crystal to timer2? If not, you are going to need to use one of the power-saving modes that leaves the crystal oscillator running, and these are not subject to the 65ms oscillator startup time.
Standby Mode
When the SM2...0 bits are 110 and an external crystal/resonator clock option is selected, the SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down with the exception that the Oscillator is kept running. From Standby mode, the device wakes up in six clock cycles.
Since most Arduino boards use a ceramic resonator for the main clock, you might also consider using a different fuse setting with a faster startup time (resonators start up faster than real crystals.)