I think RFC has already commented on this issue before, both in the textev (that Merlin wonders if any of Pei Shan-wei's sleeper agents left any written evidence like Jeremy Knowles) and in the various conferences. He stated that the only reason the Diary of Saint Zhernau survived was because of a unique set of circumstances, namely Charis' "backwater" nature throughout Safehold's history, the specific instructions he left for his followers (keeping the documents sealed away in a vault for 400 years), the one aspect in which his religious teachings differed from Church orthodoxy (a more "protestant" view of God that encourages a more open discourse) and the drop in literacy rates (which led to a greater reverence for the written word). I also think that the Ahrmahk dynasty's rise to power was also related to Knowles' brother in-law Caleb Sarmac, who may have used the knowledge given to him to establish the necessary nest eggs needed to give the family a leg-up in the future.
By the same token, only a unique set of circumstances allowed the Sisterhood of Saint Kohdy to survive and preserve his diary and "holy relics". His status at the forefront of the War Against the Fallen meant that they couldn't simply eliminate him without consequences. His sudden death in Zion raised many questions they refused to answer and simply ignoring the standard practice of canonizing the fallen Seijins was also impossible. It was only afterwards, when all the Adams and Eves who knew him personally had died, that they moved to eliminate his legacy. And his followers survived to give the Church grief only because of similarly unique circumstances - The Church's ignoring their request for funds (leading them to find their own sources of funding), the original Mother Abbess having a brother who was a vicar (who warned them of what was coming), her foresight that led her to divest enough of their resources (which allowed her to hide that money from the Church's auditors) and her successors drawing the conclusions that the Church was hiding something big where Saint Kohdy was concerned (which led them into eventual opposition to the Church).
So I think it's unlikely that there's some additional hidden caches of technology anywhere, aside from Nimue's Cave and the Temple. Given how "thorough" Chihiro and Schueler were in rooting out "heresy" and given how much of that "thoroughness" (as Staynair put to Merlin) still clung to the Inquisition until now, anyone exposed to such technology or any lost gospels or records would have been eliminated just for good measure. You would need a similarly unique set of circumstances that would allow any "rediscovered" information not influence people over time. It would need to be a record, not kept in any form of remotely discoverable technology (i.e. nothing with an energy source), hidden in a way that would allow the documents to survive the ravages of time (like inside a sealed container), and it would need to be discovered by someone open-minded enough not to hand it over to the Inquisition as heresy or destroy it on sight and strong-willed enough to do something about the inherent lie of the faith held by all on Safehold.
