Simple workflow question with alignment

Imagine a movieclip which is a balloon inflating.

You place the movieclip on the main stage. You wish to put a thumbtack on the main stage also, positioned so it touches the balloon when it’s fully inflated.

But the balloon movieclip is sitting at frame 1 where it is deflated. How can I lock the balloon movieclip to a later frame where the balloon is inflated, so I can appropriately position the thumbtack?

Thanks - this one has bothered me for ages.