Wow. While a pressurized housing may do a better job at keeping moisture out, I am not sure that they will withstand driving sand better than the MIC400.
I've seen Pelco's EH8100 work well in the field, and Videolarm released the PFH10, but I have not seen one in person. Unlike the IP68 rated MIC400, they are rated to IP67. Both feature glass front lenses, but they would be susceptible to etching and driving sand in harsh environments, too.
Really the question of IP68 vs. IP67 in this application is academic, since the difference between the two relates to immersion of 3 feet and greater. Your application does not sound like a specification rating issue; rather it sounds like the environment is just chewing the housings up!
To answer your initial question "Would a pressurized housing be a good way to go?" I would answer it wouldn't necessarily give you a better result just because it is pressurized. I will ask the folks at Videolarm your question and report out on their feedback.
I look forward to reading other replies!