Really interesting, although I have yet to give it a try since I haven't managed to kill myself yet on my current run. But a few questions/comments/ideas:
1) Can you release a separate (lighter weight) gem prospector mod that generates prospectors using the same generation code that is used for dromad merchants (since I'm assuming they are more merchants than arconauts or tomb robbers)? Plus also generate jade, but that's really a non-issue. I ask since that would allow for at least some part of this mod can be added mid-game...

2) Depending on your personal capacity for coding, can the code that is used to generate cryotubes underground be adapted to generate this settlement rather than during world-building? I understand that the cave generation portion of the game's code is mostly a separate system (I guess it only really connects via parasang tier and depth, everything after that is just the normal zone memory system). As far as I know, the game code limits cryotubes from generating on the surface by limiting them from generating in a zone tagged as a "surface" zone. However, adapting this generation method to instead occur in zones/parasangs tagged "surface" would still have some further issues to fix:
A) It would need to be hooked into the secrets/settlements system somehow, since this can give locations based on initial world generation (so if the "secret" location is found, it would need to be sure to generate where the given location is once a player arrives there; this is so since it technically won't exist until arrival).
B) It can't just generate based on a % chance per parasang tier and/or parasang type mid-game, since there is a small but non-trivial chance of it never generating (whereas it is guaranteed to generate somewhere on the surface during world-building, and then can be hooked into the secrets system). This would necessitate some sort of logarithmic progression code to guarantee eventual generation (so that over time, the chances of it being generated will increase). Put more succintly, just look at a graph of a sigmoid curve, or more probably its inverse, the logit curve; the Y eventually maxes out over the X axis (X is time for the purposes of this example), but it isn't linear progression.
C) Location generating chance and secret reveal system need to interact both directions (so doesn't matter which comes first, the chicken or the egg).
D) The location wouldn't be guaranteed (although this could have been the case anyways).

3) Curious why this mod can't include stairs down (ie, mine shafts).

4) Is it always visible when mod is loaded (like asphalt mines), or is it not revealed until arrival (like generated villages)?

Suggestion #1 is probably doable, but suggestion #2 just as likely not (even assuming you possess the sufficient coding and maths to do it). But I'm quite interested in #2's possible success, since I'm considering adding something similar once the game's code is somewhat stable (ie, 1.0 release). Food for thought!!