"like that will collect dust in megaton quantities. And you can't hard-seal the device away to avoid moisture because it'll overheat in short order."
Dust and moisture are being mentioned again and again here. I don't get that. Most dust is generated in places where humans shed skin flakes, rub clothes, and walk over carpets. Where does all that dust come from in a crawlspace? From that tiny amount of airflow? Spores of massive fungal growth? (then you have another problem to deal with first).
As for humidity: a running server will always be slightly higher in temperature than the environment. I don't see how water vapor that has been flowing along lots of cooler surfaces can condense on the server. It would only be a problem if you put the server very close to a ventilation opening, if warm humid air enters from there. Or if condensation is actually dripping from the crawlspace ceiling, onto the server.