I give up
I eventually gave up and just edited the mission to trigger the "Prevent ship from leaving" goal immediately upon start. Then the ship spawned and I could attack it immediately. With that done, I could finish the rest of the mission in the normal way. (Yay for the mission editor... :-P)
That's the only way I could get it to work, although if anybody has any good insights I'd still be interested in hearing them.
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