Hi,
I know this seems like a naive question but I think I remember that in newer versions of Casper, there was no need to have a firstboot script to kickstart ard? IIRC screensharing would be on but the check box in SysPrefs>Sharing wouldn't be ticked and only the management account would have rights to screenshare on the box? Maybe?
The reason I ask is that lately machines that are going through Casper imaging don't have ARD enabled at all...
I use AutoCasperNBI to build my NBI's and do't do anything more exotic than that.