IMO this identifier is unneeded. Not because it can be done with scripting but because it's poor scripting to need such an $id'er.
The onLoad event is for just that; when the script is loaded. This is for things such as creating a hash table and filling it with defaults, setting default variables, creating files, etc.
onStart is an event for when mIRC(and the script) starts. This is for things like making sure needed files for day-to-day use are there, loading hashtables, etc.
The ONLY reason I can see the need for a $JustLoaded identifer is for a pretty(and unneeded) echo thanking the user for using your script, and as Argv0 stated, it's quite easy to handle this with the internal scripting engine
