Here's the problem: what you think you're asking for isn't actually what you're asking for.
The sexp system is very simple, and it is quite easy to add a new sexp to both FSO and FRED. Having the sexp do what you want, though, is another matter entirely.
The component pieces of this task are all already written, save one. We have the following:
2) ai-play-dead / ai-stand-still
3) [a delay for X seconds]
Thus, it is perfectly possible to simulate an AI-controlled fighter scanning a cargo container, and in fact people have; it was done in TVWP's Unification War and a few other campaigns.
The missing link is the artificial intelligence for the fighter: the AI goal that will make it fly a certain route, without waypoints; face the cargo; avoid colliding with the cargo; sit still for a period of time regardless of whether it comes under fire
; and then resume its previous goal. And the AI system is famously difficult to work with.
This is the problem with a naive interpretation of the sexp system; it's easy to miss the fact that a sexp actually has to do something under the hood. I think the worst sexp suggestion ever was when someone, in complete seriousness, asked for a sexp to enable multiple ship docking. (This was still back when ships could only dock with one other ship. For context, when I coded that feature, it involved re-implementing the entire docking section of the code and took two weeks of intensive programming, followed by a year of bugfixing.)