I knew that I had seen something on this before, and I just found it:
gazelle's post came just a few weeks before I gave the game a rest for nearly a year, so I think I never actually played with the edts code, but I just had a peek at it.
Short version: for the procedurally-named stars (the vast majority of those outside the bubble I guess?) it generates the ID64 straight from the name. For the others (ballpark 150,000 stars) it uses a brute-force lookup table to match names to ID64s. It knows the format of the VisitedStarsCache file and can read and write it (nice!!).
Another thing I just realised tonight: as far as I can tell, EDSM actually records the ID64 of all known stars as well.
So, while I've yet to try the vsc code in the edts project, it appear to fully replace the now-broken import functionality. Better still, it ought to be much faster than the import. (Other things I've read indicate that importing only handled one star per frame update so at 60 fps you could be waiting a long time if you had visited a lot of stars, OR wanted to import a file with (say) all known stars

)