It's a question that's been tickling at the back of my brain ever since. Although my thesis aims to establish that the appreciation of mountain scenery is by no means unique to the modern (say, post-1750) era, I do think that the type of heroic, nationalistic mountaineering practised from the second half of the nineteenth century is particular to modernity. People did climbing and walking on and around mountains long before then, but individuals were far less concerned with reaching the summit for the sheer sake of doing so, and as for flags, I feel fairly certain that the idea which planting a flag on a mountaintop communicates - that of 'conquering' a mountain for one's country - certainly wasn't a feature of the early modern period in Europe.
There is perhaps one exception, which I think I probably did splutter out eventually in response to the flag-planting-podcast question, and that is the ascent of Mont Aiguille (2,085m) in 1492. Charles VIII ordered the ascent of the somewhat difficult peak (this is a fortress of a hill that requires the use of hands as well as feet to be ascended) and one of his captains, Antoine de Ville, duly did so. I've been familiar with the broad brush-strokes of the story for some time, and it came to mind during the interview as the earliest example of what might be described as a mountain ascent for the sake of reflected glory (in this case probably intended to reflect directly on the person of a king in particular, rather than a nation in general).
However, whilst reading W.A.B. Coolidge's The Alps in Nature and History (1908) I came across further details, of which I had hitherto been aware. These are, in summary, that de Ville allegedly spent three days on the summit, which he had baptised, built a small hut, and had mass said inside it. He also apparent set up three 'great crosses' on the edge of the summit meadow, that they might be seen from the valley below to prove that the ascent had really happened. Summit crosses are of course ubiquitous today in Catholic regions, but it's interesting that this ascent, ordered by an earthly king, engaged so much with the symbolism and rites of a heavenly one. The centrality of religious imagery and ideas to early modern understandings of mountains is a key argument of my thesis: it seems that whilst the ascent of Mont Aiguille had what might be called a modern flavour, in many ways it was very much of its time.
A few pages later, Coolidge relates another ascent - some 252 years later - in which the summiteers established visible proof of their success. The peak in question was the Titlis, and the climbers apparently four peasants from the Engelberg valley which the mountain overlooks. According to Coolidge,
"They planted a great pole in a hole they dug out of the ice on the summit, and tied to it two large bits of black cloth, which were well seen from the village and monastery for a long time, and served as proofs of the success of their adventurous undertaking."
I'm not sure if either of these could quite be answers to the summit-flag question I was stymied by last year. To me they still seem to be quite different from such indelible and meaningful images as, for example, Hillary's summit pose on Everest in 1953. I am not sure if the action of creating a Calvary scene atop Mont Aiguille had exactly the same semiotics of a claim being staked, or a triumph being achieved. But, still they show people leaving their mark on a mountain-top - long before the culture in which they existed would suggest the mountain-top in particular as the best place on a mountain to be.