I rebased this at https://github.com/thomcc/cargo-lichking/tree/target-flag (and deleted the println's) and it seems to work (in some fairly limited tests, but enough for my uses).

Not sure what prevented this from landing

I think the main thing I wanted to do was add tests for it, but the sheer amount of setup required to effectively test it was overwhelming. I was going to come back to do it, but then just didn't.

At this point, if it appears to be working for you feel free to open a new PR with the rebased changes and I'll merge it (and probably add some more text to the help saying that it's less guaranteed to work correctly).