This worked great. Now to figure out why!
The hardfault is occurring when I build for debug, but not when building for release. And it appears to be ten bytes into this bit of generated code in the Embassy I2C handling... 
   `_ZN13embassy_stm3210_generated114_$LT$impl$u20$embassy_stm32..rcc..SealedRccPeripheral$u20$for$u20$embassy_stm32.._generated..peripherals..I2C1$GT$9frequency17h034de864f2943702E`

Is this a "Greater Than" function? (Gotta love the function name mangling!)