Chances look good I think. I visited Penn the other day and I heard that a good number of the mechanical engineers DO go on to aerospace engineering. But besides your Zimbabwe trip (which does look great I think) you should get involved in a couple other ECs (maybe research).
If you have a minute, please chance me for Penn as well!
http://talk.qa.collegeconfidential.com/university-pennsylvania/1999086-what-are-my-chances-of-getting-into-upenn.html#latest