This is very interesting topic, and the only possibility to be close to ToA/F with a IEEE 802.15.4 radio will be to utilize the SFD (Start of Frame Delimiter) signal in hardware with a timer. This can be used for symbol timing, and synchronization. However, like DH says it can end up with complex algorithms.
Kalman filtering is a great algorithm to use for RSSI analysis.
RSSI BASED TRACKING ALGORITHMS
Enjoy,
LPRF Rocks the World
