To close this one off, having spent a day working through all my tests compensating for the 1468/1032 byte leak in the STM32F4/newlib case, I’ve just run the same tests on NRF52/newlib and they show no such leakage.
I conclude from this that the problem lies with however ST built their newlib, so nothing for you guys to worry about at all; another ST gotcha to note @dnadler.
Thanks again for all your remarkably swift support.