There's a way without scripting, not the best solution but it works.
All you have to do is check the primitive sensors box in FREDs Ship Editor.
The Missiles i've used are based on the Rockeye, and it works like a charm.
Would be cool to have either a script or a weapons flag that allows the same without dumbing down
the ships targeting abilities, though.
I also would like to know why is there a differnce in behaviour, i thought missiles fired without lock behave like dumbfires in case of flaged as ASPECT, and no change in behaviour when flaged as HEAT ( target only one ship ).
EDIT: Seems to work for the AI too ( primitive sensors have to be checked for them as well ).
Here are some screen shots, ( total missile pawnage

):

