if you need to change your academic stuff - -do it during the open academic change period. if letting them know about your research is really important... like make or break... then send the committees at each school a brief letter about what you will be/are working on.
honestly... at this point, they have probably reviewed your file already. so unless you have heard nothing from any schools I would say that it's probably not worth it. talk about it in interviews and what not, but by the time the schools get the letter, get it to your file, and actually review it by someone who matters.... it will already be dec first.
if it's a big deal to you, then go ahead and send something.... but chances are a semester long research project at the end of college is probably not going to make that much of a difference.
good luck!