SNR in distance estimation?
Signal strength is reliable to a point. Objects getting in the way can both reduce a signal and cause reflections (that in turn cause considerable fading of an RF signal). TDOA is affected by fading in that a possibly stronger signal that has taken a longer path can "win" and give a longer distance than the direct measurement.

Noise is even less reliable and is certainly not controllable. Generally, you can include "interferers" under the umbrella of noise so it's pretty unlikely that "noise" can improve the already tenuous relationship between distance and received RF signal level.

I think the method you are using (TDOA) is likely to give you the best results without trying to figure out noise levels.



EDIT - As Robherc suggests using a signpost signal is a good idea - basically it can be regarded as noise but stable noise and at a set distance so, if you can ratio actual signal against sign post signal you get a something that starts to be usable. The sign post signal has to be at the same carrier frequency and can be a pulses signal with the actual signal transmitting in-between these pulses. I think it would be beneficial to modulate each carrier with some form of baseband signal (on sign post and "actual") then demod both signals to calculate relative strength.