Never mind! After some fiddling around without much understanding of how anything works, I seem to have got it working.
Just for anyone who comes across the same problem with no clue as to what they should do, here's what I did:
Since I was already using GMLive (which if you don't already, this will be another reason you should), all I needed to do was to paste the following code in the creation event of the object I wanted the custom variables / creation code to be applied to.
room_pack_eval_script = live_execute_string;