That's helpful and better than the other solutions I've tried but it still has a couple problems, or I'm doing it wrong. First thing is that the music loop saves the instrument along with the midi. It's not a big deal to delete the instrument, but it does involve additional steps and when I'm running close to the memory capacities of the machine, I don't need to accidentally load a 1.2 GB instrument (or a Kontakt multi that is even larger). Oddly when I just did this, 3 of the 8 created new tracks and imported the instrument whereas the other 5 just went to the track. I don't know what the difference was. Second, the loops do not export with useful names. Instead I get test, test (2), test (3), test (4), etc. rather than something useful like the original track names: test (violin 1), test (cello), test (oboe), test (trumpet), etc. Another advantage of the music loops are that you do get an audio render of each of the tracks to preview (so that helps), it retains the tempo information, and each of the midi files is cut to the same length to ensure they conform (which is not the case if you use the midi data from import song data).
I did discover that I can create the requisite number of empty instruments in the current file, then go to the original file, copy it, and then paste the midi into the empty instruments, and then move the midi to correct places from there, and then delete the instruments. If the two files use the same template, then you can just do a direct copy paste. I don't think that method retains tempo information, however. (S1 also works much, much better with multiple files open than does Logic.)
I already make extensive use of versions and sketchpads, both of which are helpful in large projects if you are trying to do it all in one file. But things do get unwieldy. The problem I was trying to solve today involved trying to port between two versions of the project—though they had different templates because I used the first to sketch and then rebuilt but at some point I went back to work on this one chunk in a sketchpad. In any case, I found it no easier to do it from an earlier non-conforming version than from a completely different file.