Maybe ESP-12 has enough pins to use RTC clock?
viewtopic.php?f=13&t=605
I found a doc here: http://esp8266.ru/esp8266-pin-register-strapping/
Telling that GPIO1 or GPIO5 can be used as RTC_CLK?
What about the secondary function of XPD_DCDC as RTC_GPIO0??
By the way: There are RTC functions out with 0.9.3 SDK however it doesn't work for me (yet)..