No, this has not been implemented. If you use the current Spark sleep modes when the core resumes from sleep mode it will behave like it has been rebooted.
If you directly manage the WiFi connection and use SYSTEM_MODE(MANUAL) then your boot up time will be extremely fast and predictable.
I suggest you design your app such that you save the data in external flash, you will have to be creative in how you determine the time when you wake - you could add an I2C RTC (fairly inexpensive w/ coin cell backup - they last years). Then you can just get the time from the RTC, collect your data, push it to external flash, determine if its time to connect and publish your data, then go to sleep. Seems like this would be very efficient way of managing your power.