Thanks everyone for the valuable input!
Mic, the simple '/cue/selected/fileTarget none' is exactly what I was looking for, thank you!
Scripting doesn't come naturally to me at all, and most shows I use QLab for don't need it either.
I've built a couple of things before, but I've been reading this group and I've been learning lots of new tricks, thank you!
Sam, to answer your questions: "What are the unique properties of a cue that you want to retain while removing its target file?"
Well ... everything. When a show has been built and an audio cue needs a new audio file (eg. a shorter version of a song), I want to empty the cue until the new edit is done. All of the cue's properties: level, notes, auto-follow, pre-wait, ... are often set by that point and should be kept. Obviously, this cannot be achieved with templates.
Andy's suggestion of flagging the cue is a good one, I wasn't using the Flag function before. Ivan's suggestion of using colours, however, won't do it for me, as I use those for visual reminders (eg. sound FX are purple, stop cues are red, etc.).
However, despite having the option of using the Flag function, I'd much rather empty said cues, as the effect is much more 'noticeable', eg. a broken cue.
Also, when quickly building a QLab show for rehearsals etc., a broken cue cannot be played, and thus prevents the 'wrong' audio file from being used again.
Maybe it's interesting to give you some more context.
The situation where I last needed this was when preparing for end-of-year performances in a musical theatre school where I teach (as a freelancer).
That's 5 different performances of about 1 hour, each 'assembled' in a rehearsal of about 1,5 hour. (What's the right word for this in English? We call this process 'montage'.)
So for me, that's a race, as I know which songs will be sung, but I don't know the order, the scenes, the cues, ... beforehand.
So when I'm dragging in the tapes, playing them back, and one performer says: "Oh, we were going to use the shorter version" - which I hadn't received yet, I'd like to be able to quickly empty that cue and then carry on working (writing down the next cue line etc.).
So I'm often using QLab in a very basic way, which is why I hadn't resorted to scripting for something that seemed as 'simple' or 'basic' as decoupling a cue from its targeted file. I was honestly baffled there wasn't a simple button for this, I thought I had missed it :D
But hey - I respect all of Figure53's efforts to keep making QLab an awesome tool, and if this function was never in high demand, I understand it wasn't a priority to program it.
In any case, I'm very happy that there's a relatively simple solution I can use, and a group like this for tips & tricks. Thanks everyone for the input!