i just said what i would do, same as you did and others did.

the op said he was doing well on 0.6ml so why would you tell her to halve it?
i dose according to my vets instructions aswell. i've never been given as low as (at least initially) that by my specialist vet and any other vet who has worked there. i can pass on the info and advice i think is right, same as you. it's upto the op and their vet what they then decide is the best for that pet and their condition.
even if metacam is effective pain relief wise at 0.2ml, that doesn't mean the inflammation doesn't need higher than that to bring it down and under control, at least initially. there is data available on the use of metacam in guinea pigs which states this that vets can have access to.
i'm sure sarah pellett will beable to work out what is going on and what doses will be best in this particular case anyway and knows much more than either of us ever will