Hey mate,
You could create an invisible dialog, and then createdialog it when the cutscene starts. When the player hits ESC, it will close the dialog, which will cause the
dialog command to return FALSE. Knowing that, it is just a matter of putting in the proper checks/gotos/whatever into your cutscene, and you're good as gold.
Only problem with said method is that it would cause the mouse cursor to appear during the cutscene. Not really a huge problem, but worth noting.
On a side note, I have suspected that there are certain 'hidden/secret' global variables that BIS never told us about, but that tell us stuff like when the player presses certain keys. My evidence is in the "camera.sqs" script (found in DTA/Scripts.pbo), there is a line that waits for the variable "cheat0" to change before terminating the camera. There is nothing in the script that changes that variable, so perhaps pressing a certain key would change it. Its all speculation, but someone may be inclined to research it further.