One way around this is to set the high-salience "default" rule to also trigger on the non-existence of some "marker" object. Then, in the consequence, you can always assert a new instance of that object, which then effectively blocks the rule from triggering again. If you want it to automatically manage itself, you may be able to do an assertLogical on the marker object and then it will get removed when the business object gets retracted.

So, something like this maybe:

rule "Pre"
salience 10
when
$bo: BusinessObject(targetDate != null)
!Marker()
then
assertLogical(new Marker());
Date tmp = null;
$bo.setTargetDate(tmp);
modify($bo);
end

I've had to do that sort of stuff before to handle edge cases like this.

--- Michael