Sounds like you're having fun over there, with this issue and now the flooding!
I second the idea of getting her a goaty companion; I personally recommend a goat around her size, or slightly smaller (so she can't hurt it accidentally), without horns. A wether would be best, as it won't go into heat. My one boy's in with an older nubian whether, because he can't play nicely with other horses!
As for her eating sawdust... does she have a balanced diet, and is she getting all the minerals she needs? She may be looking for something she needs. If after switching to straw, she moves on to something else (like cribbing, or finding another way to eat wood), offer her more minerals, and maybe have a blood test done.
I've never heard of a horse doing this, but admittedly, I am not a horse expert. A family member, years ago, had a dog who would eat dry wall, no matter what they did; it turns out the dog had a rare blood disorder (it couldn't produce red blood cells, I think; I'll need to ask what exactly she had), that was the result from generations of poor breeding and inbreeding. She simply needed a mineral the drywall had. Her colicing may mean she has another disorder, unless it is stress related, or the sawdust did it.
The only other thing I can think of is, was she properly fed?
I got a mare a while bad, that was INCREDIBLY underweight, and had never been allowed to eat enough. When I first stalled her, she started eating the bedding, until she was given feed and hay. For a few days, she'd start doing this if she ran out of hay, but she's now stopped; she knows food will be there, and she won't starve.
Best of luck!