I had my freehub refuse to engage during one commute last year toward the end of winter. It effectively brings your ride to a halt (unless the remainder of your ride is downhill). This was likely caused by condensation due to the fact I use indoor parking at the university.
Has anyone else had this happen? Any tips for preventing this or dealing with frozen freehub pawls should it occur on the road?
If it's a Shimano freehub then it's possible to take it off and dissassemble it, clean the insides out, put some very light grease on the bearings and some light oil on the pawls and then assemble it back together. Most people don't do this and it's not reccomended by Shimano.. but it can be done. The easiest thing to do is to buy a new freehub body, they're not very expensive.
I've had it happen with a Shimano freehub. I didn't totally disassemble the freehub; I just removed it, and pulled the back gasket. Then I used tons of WD-40 on it to displace the water. I blew it out with an air compressor, then let it dry. The next morning I squirted oil in, and replaced. That was 1 1/2 winters ago (the first winter on the hub), and I haven't had any problems since.