You will also note that this law (or rather these offences) are related to the RTA.
There would be no alternative charge with this, why should there be, if you are guilty of section 92, then you will (because of the low BAC limit ) be guilty of S93. This is assuming Alcohol is involved not drugs. However if you are guilty of S93, then you may well not be guilty of S92.
There is a power to breath test for S92, but there is no requirment to do so. The offence is Working while unfit. The evidence of being unfit does not have to be the BAC level its the person as a whole and what he is, or is not doing. Whith vehicles, usualy I only used that for someone who was what amounted to drunk. In any case a person arrested under this section would be examined by a Doctor at a Police Station, and that would add weight to the evidence.
With regard to drugs under S92, thats never going to be revealed by a Breath test. Again the evidence is what the Police officer sees hears and possibly finds if he searches the person. Remember that drugs can be any substance (except alcohol)So some cold medicines can make you unfit though drugs.
The reason I said be careful with this is because the way you put it could be interpreted as Police will Breath test if someone is arrested for an offence under S92. They probably wont. Although it has to be said, I would always go for S93 and breath test, its easier to prove.