Thanks for the suggestions. I'm not sure exactly where my meeting is, but my college said it would take about 1/2hr to 45 min from the airport. I tried doing 2 separate tickets (and nearby airports), but it still kept kicking the fare out of whack when I put in more than a 4 hour stay in IAH. I can get a single ticket on AirTran for $223, plus ~$200 for a MDW-LAX on another LCC, but then I have to stay in CHI for the night, and either pay for a hotel or drive back and forth like crazy. I stay at a friend's house in LAX. Also, no chance for NW miles (even at 50%, I still want them) or upgrades that way.
I checked the availability and it is good for the later flights IAH-LAX (for 1/19, and for this Wed 1/12 for comparison); generally the later the better, so I guess I'll take my chances. Can't cost me any more than the $700 fare difference for booking it later on purpose.
One last question- when I check in for the flights in MDW, it will print out both boarding passes, so if/when I go awol in IAH, will this cause problems for me showing up for a later flight?