So in the absence of braking action reports, is there another way to judge what the expected braking will be?
I would say, no... Any rule of thumb will be worthless, since there are other factors. Like, rubber deposits in the touchdown zones, length of runway (speed when entering opposite touchdown zone) etc. braking action will probably vary during the landing rollout.
Hence, local knowledge combined with a carefull though through performance calculation is probably the best answer.
Same goes for takeoff performance - runway wet and heavy rain. Reduced performance takeoff combined with a reject may have you braking on wet rubber deposits. So if you know the state of the normal dry runway, you may have a better possibility to make a good decision.
For both takeoff and landing calculations, slippery/braking action good will probably be more conservative than just wet. Plus a good margin on performance for takeoff and higher brake setting for landing, to ensure low speed in the opposite end may be a good idea in my opinion.