try the heterocedastic robust Lo estimator (by andrew lo)

try this paper, it gives some clues on the process:Real-Time Trading Models and Statistical Properties of Foreign Exchage Rates (1998)

Regading FX tick-by-tick trades, a conditional probability of tick signs is something that can be exploitable e.x. P(+|--) >> 50%, and things like this...

After 3 long years did you get any progress on the subject ? I´m interest in quant approachs to general equity valuations....