hi, jwayand.
the other day I posted a reply to a similar question in the DirectRT section of this forum.
http://support.empirisoft.com/showthread.php?t=8169
if you have Direct RT, and still need to stop a video via a key press, record the reaction time, and proceed to the next part of the study, then you may want to review my example, you can download at the link above.
if you do not have access to Direct RT, and still need help with this in MediaLab, please let me know, and I'll look into it