Indirect speech: Can you express that you don't believe the original speaker of a sentence (with the help of a tense or a verb form)?
When reporting a speaker's words about the present or the future, we can choose to backshift into the past or to keep the original speaker's tenses. For example:
I told my boss I will/would be late to work tomorrow.
He asked me how old I was/am.
But as Swan in Practical English Usage (p277) notes:
We are more likely to change the original speaker's tenses if we do not agree with what he/she said, if we are not certain of its truth, of if we want to make it clear that the information comes from the original speaker, not from ourselves.
Two of Swan's examples are:
She just said she was fourteen! I don't believe her for a moment.
He announced that the profits were higher than forecast.
However, this neither implies that using the original speaker's tenses means you believe what they say, nor that backshifting the tenses means that you don't. Generally, the context should make it clear where you stand.
No. If this were possible, it would have to be with the past subjuctive of to be, viz were.
It should certainly be understandable, if you were to say
He said that he were finished
that by substituting were for was you meant to suggest that he were not in fact finished. But it will not generally be recognized as correct. Also, it would be very conspicuous with a main verb in the present tense:
He says that he were finished
So no, it would be streching English beyond its capabilities. It would be interesting, though, to see if there are any historical examples of this.