? Strange... I thought that if you combine dotarget with dofire it would work... Anyway, have you tried to combine _unit DoTarget _target with _unit Fire ["RpgLauncher","RpgLauncher"]... I can't promisse you anything, I couldn't make it work either...
:-\
But is it for a cutscene? If so, why don't you CamCreate the rocket at the soldier's position, towards the target...? It would require a litle of trignometry, but it would work.
And... what if you set your target to enemy (at mission.sqm), set it captive (init: this setcaptive true) and set it enemy again (this setcaptive false) when you want your soldier to destroy it...? Once again, I can't promisse you anything.