I think stamps for SPQR type "property of the state" were common. But they are an ideogram, a marker. No different to the writing on a coin in that regard, for illiterate people. You just recognise it as a whole, not as a meta-notation for more complex ideas.
The building blocks aren't enough. You need the next step which is the motivation to innovate. At the other end of the trade line into Asia, they were working up to printing in the mid-first millenium, which overlaps with the END of the roman empire. Movable type came later.
Before movable type, I believe "chap books" were being printed from woodblocks for ordinary people (at least ordinary protestants!) which probably encouraged widespread literacy, something which had dropped out of society when late roman empire converted over to serf economies, and didn't need ordinary people to even think about being literate. You weren't moving an army off the local patch nearly as much, or coordinating reports in one mother tongue back to head office by courier.
(this is probably a classic "bad history" take which deserves a "well... AKchewally..." from somebody like prof. Mary Beard.)
