From "Reducing the Risk of Human Extinction", by Jason Matheny. (http://wilsonweb.physics.harvard.edu/pmpmta/Mahoney_extinction.pdf)
Colonizing space sooner, rather than later, could reduce extinction risk (Gott, 1999; Hartmann, 1984; Leslie, 1999), as a species’ survivability is closely related to the extent of its range (Hecht, 2006). Citing, in particular, the threat of new biological weapons, Stephen Hawking has said, "I don’t think the human race will survive the next thousand years,unless we spread into space. There are too many accidents that can befall life on a single planet" (Highfield, 2001). Similarly, NASA Administrator Michael Griffin (2006), recently remarked: "The history of life on Earth is the history of extinction events, and human expansion into the Solar System is, in the end, fundamentally about the survival of the species."
Can anyone point me to existing thought on space colonization from an EA perspective?
You're welcome! :-)
Whether this is indeed a dissenting view seems unclear. Relative to the question of how space expansion would affect x-risk, it seems that environmentalists (of whom there are many) tend to believe it would increase such risks (though it's of course debatable how much weight to give their views). Some highly incomplete considerations can be found here: https://en.wikipedia.org/wiki/Space_colonization#Objections
The sentiment expressed in the following video by Bill Maher, i.e. that space expansion is a "dangerous idea" at this point, may well be shared by many people on reflection: https://www.youtube.com/watch?v=mrGFEW2Hb2g
One may say similar things in relation to whether it's a dissenting view on space expansion as a cause (even if we hold x-risk constant). For example, space expansion would most likely increase total suffering in expectation — see https://reducing-suffering.org/omelas-and-space-colonization/ — and one (probably unrepresentative) survey found that a significant plurality of people favored "minimizing suffering" as the ideal goal a future civilization should strive for: https://futureoflife.org/superintelligence-survey/.
Interestingly, the same survey also found that the vast majority of people want life to spread into space, which appears inconsistent with the plurality preference for minimizing suffering. An apparent case of (many) people's preferences contradicting themselves, at least in terms of the likely implications of these preferences.