I don't know of many examples of this except the ones you mentioned. I do have some personal views on the matter though. Drawing from long, intense, jumbled thoughts on the subject and a smidgen of psychology... here we go:
What motivations would citizens of an ultra-high technology society plausibly have besides boredom? And how would lower tech beings meaningfully interact with them?
I'm going to look at this from an immortal perspective. If you lived forever and your trying to stave off boredom your either:
You could dabble in the middle of the two extremes but if you live long enough even the dumbest person will learn everything just from walking through the universe. So now the question becomes what does a near-omniscient near-immortal being do to pass the time? Once again you either push the boundaries or play the odds. But go far enough and maybe you hit the wall and know how things will play out. What do you do when you've done everything and know all the certainties? You watch the racehorses and dare each other to jump off the skyscraper. There has to be a limitation of your abilities. If there is you'll let chance wiggle into that niche and make a game to amuse yourself, only the unknown will be novel at that point, or maybe you'll fade from existence after having your gazillion-th burger. Maybe you seal some of your ability or bet with it? At some point you can roll the dice and not know the outcome. Maybe there is no limitations, or maybe you just prefer the alternative? How about a battle of wits? Maybe with the low-techs as the RNG? But when gods wage war its bound to be bad for the universe. Either way things like humans end up looking like Honeybees. Social, maybe a source of enjoyment, maybe a source of displeasure or pain, maybe even necessary for the fabric of life as we know it, but ultimately insignificant.
So what about that other way around? How do the cave peoples interact with the aliens?
Why would the HPLD culture allow them to? If they already are having a hard time of it maybe they aren't talking out of goodwill. Why hasten our demise? If you become a god and can resurrect the dead then wouldn't it eventually happen? Wouldn't it look like Walt Disney never died, when to them he was only dead for a tiny fraction of the time of his immortality? If nothing is of consequence then why hasten anything at all? Whimsical, lazy, or mocking views on our fate seem like the most likely outcomes for how the culture would view any low-tech culture. If they're not being neutral out of the kindness of their hearts or because they don't care, then their doing it out of some vindictive spite. Of course if they killed everyone they might save them from the endless boredom of immortality.
Of course this is at the top of the power spectrum. As you start to come back down things can get interesting again. Like your Babylon-5 and Dancers at the End of Time examples. If the culture isn't dead then you have time to develop a culture around doing whatever the hell you want. Probably the main rule would be "don't piss on another immortal's parade." (let's hope we're somebody's garden project). Ultimately you end up at the top of the spectrum unless the universe dies before then. On the flip-side, given infinite time everything is plausible. So either set a death date for the universe and make a culture set out to have a blast and do whatever the individual wants, or pick whatever culture you want to have and pick whether it devolves into whimsy, laziness, or spite, spirals off through the culture continuum, or cycles back on itself stably.
TL;DR They either are having fun still and maybe we're part of the game, or they're not and no one would want to join them because they're either insane or depressed. The first isn't likely to invite a bug to the party, and the second would squish it (or we'd have wished they did).