...one could postulate that maybe there was a spacegoing Klingon civilization that preceded Kahless, one that later regressed.
Or one could say that Klingons were quite starfaring before Kahless - but only when star-transported by their Hur'Q/Karsid masters.
These pre-Kahless Klingons might not count as people from the Klingon Empire yet, but the distinction wouldn't matter for the purposes of the
Children of Kings line. (What was that line, anyway?)
I don't recall any mention of warp drive driven Bajoran ships either in the present or past.
Several Bajoran-operated ships went interstellar in DS9 - the first to explicitly do so was the "scout" in "Past Prologue", its model later reused as a military and civilian transport that also performed interstellar journeys. We never actually saw these ships to go warp, but we did see them sport what looked like conventional warp nacelles... So, some sort of conventional-looking FTL tech was available to Bajor immediately after the Occupation. What they had before it remains speculative.
In fact,
TNG: "Ensign Ro" said that Bajorans had been a spacefaring civilization .. [..] DS9 established that Bajor only had "primitive" solar sail craft a mere 800 years before the series.
That's not quite accurate. The TNG episode did not make any mention of spacefaring let alone starfaring, merely of advanced architecture and society.
DS9 "Explorers" in turn said that the sailships existed 800 years prior to the episode and were used for exploring the Bajoran system, but it doesn't establish that they would have been the only thing that existed. Many people enjoyed exploring the Earth's bodies of water in sailing craft in the 1960s; this in no way precludes the existence of supertankers, nuclear submarines, hovercraft or hydrofoils at the time, nor negates the expeditions of Cook or Zheng He long before that date.
"Explorers" also discusses the concept of ancient Bajorans visiting Cardassia in such craft. We get no clear timeframe for that, though. Perhaps it happened 20,800 years earlier, rather than 800? Lightsails (of all sorts of designs) might have remained fashionable for the intervening years, only going extinct on that significant day 800 years before the episode.
That's the gist of Sisko's interest, after all: not that the lightsail design is an amazing 800 years old, but that these ships have not sailed for the past 800 years and have been largely forgotten. The history of Bajoran lightsails might extend back several thousand additional years.
Timo Saloniemi