Hey guys, just thought I'd clarify here for more details. After submitting your original application through TMDSAS you actually cannot send in an update to TMDSAS regarding recent activities, interest, or intent. There is no way for TMDSAS to selectively distribute this information. You can send these updates to the schools themselves, but the school may or may not opt to take that information into consideration. From what I know, UT Houston, for example, hates updates and completely disregards them.
You also could send TMDSAS an updated transcript. They are able to officially send that information to the school, and the schools will officially receive those updates. This works pretty well with summer courses, but because for most schools in TX the interview season ends in December, it doesn't really make an impact for fall courses.