More about hacc as it is currently (2017-03 +-):
The survivalfly check (+-abstraction) provides a "base speed", which only is about what a player can run at without using special technique like extra jumping, i.e. not necessarily the finally allowed speed. For hacc we feed 1.0 whenever we're below the allowed base speed, and actual/base if we're above (correct me if i am mistaken). Roughly all 20 or 30 moves we check where we are at, also think of some resetting logic existing and possibly workarounds for medium entry with higher speed. At least something like that is the conception. This way we don't need to account for short-term specialties like double-bunny and bunny-ho and so on, but we can limit the average thing.
This seems to be somewhat effective, haven't tried to push the border to the lowest thinkable thing, yet - there may be improvements taking account for height differences of the environment or what not, or to have both a shorter term and a longer term observation thing, to ensure bursts only happen now and then (provided that exists at all). Currently a fixed period of 20 or 30 moves is regarded (based on move events, not time), having two periods of different length covered could give a slight boost (about to test that).
Apart from improving the mechanism, there also could be other side kicks, see NoCheatPlus/Issues#418 for related examples.