You can do this with the function object. Call a function, eg. "DoStuff" and forget picked objects. Then add an event like:
+ On function "DoStuff"
+ Sprite UID is 2
: Sprite: do something...
Because the picked objects are wiped clean for the function call, you can then pick the sprite with the UID you want from a clean slate and do something with it. Remember, after the function call the picked objects from the old event are restored <img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" />
If the object of the UID you want to specifically modify is already picked, you can add a subevent 'Sprite UID = 2' and do something with it there. It will then pick the sprite with UID 2 so long as it was picked in the parent event.
Out of curiosity, why do you need actions on specific instances? Just wondering the circumstances, in case we can better design a solution (or if one already exists).