In the sentence
- Only now do we have what we need to move forward
the word do clearly has some emphasizing meaning.
Let's clear the mud off the sidewalk first.
The final clause is irrelevant, so an equally good example of the construction in question is
which has three syntactic features:
- fronting of only now
- Do-Support to provide an auxiliary do
- subject-auxiliary inversion of we and do
Notice that the only is necessary. If we omit it, the sentence is ungrammatical
and instead we would say
with fronting of now, but no subject-auxiliary inversion and no Do-Support. This is because you don't need an auxiliary if you aren't going to do something with it, and ordinary adverb-fronting does not govern subject-auxiliary inversion, so points (2) and (3) above don't apply.
But only now is a negative adverb of time, meaning that in all the time before now, the sentence was false, so that we didn't have it. As it happens, when a negative adverb of time, place, or circumstance (one that would negate the whole sentence) is fronted, subject-auxiliary inversion is required, and if there isn't already an auxiliary verb, Do-Support is also required, so that's where they come from in this sentence.
It's not true, though, that do has any meaning in this sentence, let alone an emphatic one.
What catches your attention is the odd word order -- the
Quantifier Adverb do
Pronoun Verb Pronoun of Only now do we have it
instead of just the normal
Adverb Pronoun Verb Pronoun of Now we have it.
Use of auxiliary do is part of that pattern, but do itself has no meaning at all.
Like the there of There-Insertion and the it of Extraposition, do is a Dummy,
put there to fill a slot in the syntax and governed by strict syntactic rules.
The emphasis that you perceive is a result of the constructions, not of any particular word.
Grammar is constructions fitting together, not words on a string.