Are successes at the IMO a reliable and objective measure of the skills you need?

Well, of course, this is a major filter for intelligence, creativity and plain math "basic front kick" proficiency. However, you also exclude lots and lots of people who may possess the necessary skills but did not choose to enter IMO, were not aware of it, had other personal commitments or simply procrastinated too much etc. Also, the skills IMO medalists have acquired so far are in no way a guarantee that more skills will follow, and may be the result of good teachers or enthusiastic parents. Let me present you some weak evidence, behold, a personal anecdote!

On behalf of my chemistry teacher and owed to my school's policy in general, I entered one of these sciency olympiads a while ago. I procrastinated over the first round, which was a homework assignment, got it in just in time not quite completed, but was allowed to the next round. From there on, I made it to the national team and won a medal at the international competition. Looking back on what I did back then, I'd say the questions were quite easy, not at a level I'd call requiring serious skill. Of course I've continued to learn much since then, but it could've gone another way. I would not say such medal winners are overly altruistic or more determined to their cause than others are (the two of my teammates that I know about are now studying medicine, "to make money" as they told me).

I'd conclude that if you want bright mathematicians, you might be well off taking IMO medalists. But if you want people who are also trustworthy, altruistic and deeply committed to AGI, let alone especially rational, you should probably widen the filter for intelligence and math skills (if IMO medals are a good measure of this) a lot. Perhaps ask Mensa, take applications and filter from those, reach out to the best 1% on some college entry test or something. Just wild, uneducated guesses. But focussing on IMO medalists, which should give you less than 500 potential candidates a year, doesn't sound like a good strategy. More so since those people are approached not only by you but also by quite a few companies.

Perhaps I misunderstood what you mean with "at medal-winning level", but since you're doing this SPARC camp and I can't think of another way of attracting such people, I assumed you were reaching out to people who actually compete.