Could I use "setScript" on some ship in the system (say, the main station) to briefly set the script, run the function and then reset it?
I would strongly advise using a disposable entity - hide an alloy plate at 10^13 metres from the sun, or something - rather than anything which might have a meaningful script on it, for that sort of trick. Resetting the script afterwards won't reset the values of any variables (which can include event handlers, of course) changed since load, and nor will it re-run
or similar events if that's needed to set up other parts of the script.
Anyway, that would work in terms of granting access to the condition script's initial method definitions, and in many cases that's enough, but won't work in the general case because the condition script might have its own internal state, and creating a copy of it won't copy that internal state.
Condition scripts could also have conditions based on external environment - time, mission variables, state variables in other scripts, who else is currently in-system - so if you're doing this for the traffic reports, you might find that the condition script reports one value when you try to schedule a launch for a few days time, and another value when you actually get around to launching it.