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
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).