Not quite, Cram, because pouvoir is a special case.
At the same time, cram1993 is right, in the sense that all verbs were like
pouvoir in earlier stages of French. As you and Aupick have both explained, in
ne dit mot, the negation is expressed by
ne alone.
Mot reinforces the negation, but its primary syntactic function is realizing the direct object of the verb
dit. In other words, it corresponds more closely to
rien than to
pas. It was never possible to say, for example,
Il ne dit mot son nom to mean something like
Il ne dit pas/point son nom.
The examples he gives are all from Low Latin or German, so I guess he means the Early Middle Ages, around 600-1000 AD.
Not earlier than the 12th century for French.
did you know that
je n'y vois goutte was originally copied from
je ne bois goutte - I don't drink a drop 
?
I know that many people believe this, but there doesn't seem to be any good evidence for it. The usual assumption is that
ne boire goutte became
ne voir goutte because the verbs sound so similar, but in fact the verbs sounded completely different in Old French (
beivre vs.
vedeir), except in the 3rd sing. pres. ind. (which is admittedly a frequent form). More problematically, according to Price (
1990,
1997), there are no examples of
goutte used with
boire in any Old French texts. His alternative proposal is that
ne voir goutte is a variant of another attested expression,
ne voir larme (i.e. "I can't see the tears in my own eyes, it's so dark").