Time remaining with os_timer_t
The library just calls os_timer_setfn and os_timer_arm which will call the callback function after some time. To add the feature to see how many remaining time there is, you would need to extend the library (the Ticker class, specifically) to save a timestamp when the timer was started. You can then write a function which returns for how much time is remaining or has passed by doing uint32_t remaining = timestamp_now() - timestamp_started. 

A good function to hook would be the _attach_ms in Ticker.cpp. Add a uint32_t timestamp_started variable in the Timer.h header for the class. Then set timestamp_started = system_get_time() (from user_interface.h). Then write a function uint32_t remainingUs() { return system_get_time() - timestamp_started; }. Beware of integer overflows -- system_get_time returns passed microseconds since µC startup.