I realize this might just be a no-go, but it would certainly add a percieved value to the Propeller 2.
I recently came across the DS1672 which is a 32bit RTC I2C solution for the Propeller 1 or maybe the 2. But the cost is about $2.00 or so per chip.
Having this feature, or all the features of the DS1672 internally would appeal to many developers. It just might be a deal-making feature.
Take a look at what it does. It seems like the core feature would just take a wee bit of silicon, and of course two i/o for the clock crystal and one power for a backup battery.